In situ bioreactors and deep drain-pipe installation to reduce nitrate losses in artificially drained fields

Existing wetland conservation programs miss nutrient reduction targets

Restoring wetlands will reduce nitrogen contamination from excess fertilization but estimates of the efficacy of the strategy vary widely. The intervention is often described as effective for reducing nitrogen export from watersheds to mediate bottom-level hypoxia threatening marine ecosystems. Other research points to the necessity of applying a suite of interventions, including wetland restoration to mitigate meaningful quantities of nitrogen export. Here, we use process-based physical modeling to evaluate the effects of two hypothetical, but plausible large-scale wetland restoration programs intended to reduce nutrient export to the Gulf of Mexico. We show that full adoption of the two programs currently in place can meet as little as 10% to as much as 60% of nutrient reduction targets to reduce the Gulf of Mexico dead zone. These reductions are lower than prior estimates for three reasons. First, net storage of leachate in the subsurface precludes interception and thereby dampens the percent decline in nitrogen export caused by the policy. Unlike previous studies, we first constrained riverine fluxes to match observed fluxes throughout the basin. Second, the locations of many restorable lands are geographically disconnected from heavily fertilized croplands, limiting interception of runoff. Third, daily resolution of the model simulations captured the seasonal and stormflow dynamics that inhibit wetland nutrient removal because peak wetland effectiveness does not coincide with the timing of nutrient inputs. To improve the health of the Gulf of Mexico efforts to eliminate excess nutrient, loading should be implemented beyond the field-margin wetland strategies investigated here.

A review of ecosystem services from edge-of-field practices in tile-drained agricultural systems in the United States Corn Belt Region

Edge-of-field management practices that reduce nutrient pollution from tile drainage while contributing habitat and other ecosystem services are needed to enhance agricultural systems in the US Corn Belt Region. In this review, we identified edge-of-field and catchment scale agricultural conservation practices for intercepting and treating tile drainage. The reviewed conservation practices were (1) controlled drainage, also known as drainage water management (USDA-NRCS Code 554); (2) drainage water recycling (USDA-NRCS Code 447); (3) denitrifying bioreactors (USDA-NRCS Code 605); (4) saturated buffers (USDA-NRCS Code 604); and (5) constructed or restored wetlands designed for water quality improvement (USDA-NRCS Code 656) herein referred to as water quality wetlands. We examined 119 studies that had information on one or more of the following ecosystem services: water retention, water quality improvement (e.g., nitrate, phosphate, sediment, or pesticide retention), wetland habitat (for birds, aquatic organisms, and pollinators), crop yield improvement, and other benefits (e.g., recreation, education, aesthetic appreciation, greenhouse gas retention). We found the five edge-of-field practices were all effective at removing nitrate with varying degrees of other potential benefits and disservices (e.g., greenhouse gas production). Drainage water recycling and water quality wetlands have the potential to provide the most co-benefits as they provide surface water systems for capturing surface flows in addition to tile drainage while also potentially providing habitat and recreation opportunities. However, the following research needs are identified: 1) the disservices and benefits associated with drainage water recycling have not been adequately evaluated; 2) surface flow dynamics are understudied across all reviewed management practices; 3) a complete accounting of phosphorus species and flow pathways for all management practices is needed; 4) field evaluations of the habitat benefit of all management practices are needed; and 5) greenhouse gas dynamics are understudied across all management practices. While all management practices are expected to reduce nitrate loads, addressing these knowledge gaps will help inform holistic management decisions for diverse stakeholders across the US Corn Belt.

Improving accuracy of quantifying nitrate removal performance and enhancing understanding of processes in woodchip bioreactors using high-frequency data

Woodchip bioreactors have gained popularity in many countries as a conservation practice for reducing nitrate load to freshwater. However, current methods for assessing their performance may be inadequate when nitrate removal rates (RR) are determined from low-frequency (e.g., weekly) concurrent sampling at the inlet and outlet. We hypothesised that high-frequency monitoring data at multiple locations can help improve the accuracy of quantifying nitrate removal performance, enhance the understanding of processes occurring within a bioreactor, and therefore improve the design practice for bioreactors. Accordingly, the objectives of this study were to compare RRs calculated using high- and low-frequency sampling and assess the spatiotemporal variability of the nitrate removal within a bioreactor to unravel the processes occurring within a bioreactor. For two drainage seasons, we monitored nitrate concentrations at 21 locations on an hourly or two-hourly basis within a pilot-scale woodchip bioreactor in Tatuanui, New Zealand. A novel method was developed to account for the variable lag time between entry and exit of a parcel of sampled drainage water. Our results showed that this method not only enabled lag time to be accounted for but also helped quantify volumetric inefficiencies (e.g., dead zone) within the bioreactor. The average RR calculated using this method was significantly higher than the average RR calculated using conventional low-frequency methods. The average RRs of each of the quarter sections within the bioreactor were found to be different. 1-D transport modelling confirmed the effect of nitrate loading on the removal process as nitrate reduction followed Michaelis-Menten (MM) kinetics. These results demonstrate that high-frequency temporal and spatial monitoring of nitrate concentrations in the field allows improved description of bioreactor performance and better understanding of processes occurring within woodchip bioreactors. Thus, insights gained from this study can be used to optimise the design of future field bioreactors.

Cost effectiveness, nitrogen, and phosphorus removal in field-based woodchip bioreactors treating agricultural drainage water

Nitrogen (N) and phosphorus (P) losses to surface and coastal waters are still critically high across Europe and globally. Measures to mitigate and reduce these losses are being implemented both at the cultivated land surface and at the edge-of-fields. Woodchip bioreactors represent a new alternative in Denmark for treating agricultural drainage water, and the present study-based on two years of data from five Danish field-based bioreactors-determined N removal rates varying from 1.49 to 5.37 g N m^-3 d^-1 and a mean across all bioreactors and years of 2.90 g N m^-3 d^-1. The loss of phosphorus was relatively high the first year after bioreactor establishment with rates varying from 298.4 to 890.8 mg P m^-3 d^-1, but in the second year, the rates ranged from 12.2 to 77.2 mg P m^-3 d^-1. The investments and the costs of the bioreactors were larger than expected based on Danish standard investments. The cost efficiency analysis found the key issues to be the need for larger investments in the bioreactor itself combined with higher advisory costs. For the four woodchip bioreactors considered in the cost efficiency analysis, the N removal cost was around DKK 350 per kg N ($50 per kg N), which is ca. 50% higher than the standard costs defined by the Danish authorities. Based on the estimated costs of the four bioreactor facilities included in this analysis, a bioreactor is one of the most expensive nitrogen reduction measures compared to other mitigation tools.

Nitrate removal rates, isotopic fractionation, and denitrifying bacteria in a woodchip-based permeable reactive barrier system: a long-term column experiment

This study evaluated the effectiveness of using organic carbon materials (i.e., woodchips) to remove nitrate from groundwater. The results of our flow-through column experiment, which was conducted over 1.6 years, suggested that denitrifying bacteria reduce nitrate by using it as an electron acceptor and woodchips as an electron donor. The nitrate removal rates were sufficiently high (0.39-1.04 mmol L-1 day-1) during the operation of the column. Denitrification process was supported by fractionation of nitrogen and oxygen isotopes (δ15N and δ18O), with the δ15N and δ18O values enriched from 7.4‰ and 22.3‰ to 21.2‰ and 30.4‰, respectively. Enrichment factors ([Formula: see text]) for 15 N and 18O were calculated using the Rayleigh fractionation model, with values of - 13.2‰ for ε15N and - 7.1‰ for ε18O. The fractionation ratio of 15 N to 18O was 1.9:1, confirming denitrification. The most abundant bacterial genera in the soil used for inoculation were Enterobacter (86.7%), Nitrospira (1.8%), and Arthrobacter (1.5%), while those in the column effluent were Macrococcus (37.1%), Escherichia (14.7%), and Shigella (14.6%), indicating that bacterial communities changed in response to geochemical conditions in the column. This study suggests that nitrate in groundwater can be effectively removed using woodchip-based passive treatment systems and that information on isotopic fractionation and denitrifying bacteria can be key tools to understand denitrification.

Field evaluation of four denitrifying woodchip bioreactors for nitrogen removal in eastern South Dakota, United States

Edge-of-field practices such as denitrifying woodchip bioreactors can be used to improve the water quality of agricultural effluents. This study evaluated the effectiveness of four field-scale woodchip bioreactors in removing nitrate‑nitrogen (nitrate-N) from subsurface drainage in eastern South Dakota. Four woodchip bioreactors were installed and monitored between 2014 and 2016 near Arlington, Baltic, Hartford, and Montrose, South Dakota. Results showed that reduction in nitrate-N concentration for the four bioreactors ranged from 7 % to 100 %, corresponding to removal rates of 5 to 27 g N/m³/day for the four bioreactors during the study period. Average Nitrate-N load reduction in the four bioreactors studied ranged from 39 % to 89 % during the study period. Reduction of nitrate-N in the four bioreactors decreased, on average, by 30 % when temperature dropped below 12 °C during the study period. Flow rate and hydraulic retention time (HRT) also influenced nitrate-N removal in the bioreactors as samples collected immediately following rainfall events showed high nitrate-N load removal compared to samples collected later after the rainfall events during the study period.

Nitrate losses and nitrous oxide emissions under contrasting tillage and cover crop management

Agroecosystems in the upper Mississippi River Basin are highly productive but often contribute to deterioration of water quality and greenhouse gas emissions. Cover cropping and no-till are conservation strategies implemented to reduce the environmental impact of these agroecosystems. However, using multiple strategies can lead to systemwide interactions that are not fully understood. These interactions can affect not only environmental quality metrics, such as subsurface drainage nitrate losses or nitrous oxide (N₂ O) emissions, but also may influence crop production potential. A field trial was initiated comparing nitrate losses, N₂ O emissions, and crop production under systems with fall chisel plow tillage, fall chisel plow tillage with an oat (Avena sativa L.) cover crop (CP-oat), no-till (NT), no-till with a rye (Secale cereale L.) cover crop (NT-rye), and NT with zero N fertilizer. Pathways for nitrate losses and N₂ O emissions did not appear linked and were not tied to cover crop or tillage practices. Nitrate losses were linked with drainage volumes, and cover crops and tillage had limited effect on cumulative drainage volumes. Notably, NT-rye altered the relationship between drainage volume and nitrate losses by reducing nitrate concentrations, lowering nitrate losses by 59 ±9% compared with CP-oat and 67 ± 9% compared with NT. Neither cover crop nor tillage consistently affected N₂ O emissions or crop yield. Rather, N₂ O emissions were closely tied with fertilizer N application and seasonal weather patterns. These findings indicate that nitrate leaching and N₂ O emissions are regulated by separate mechanisms, so conservation management may require stacking multiple practices to be effective.

Evaluation of pollution swapping phenomena from a woodchip denitrification wall targetting removal of nitrate in a shallow gravel aquifer

Woodchip denitrification walls offer a potentially useful way for passive in situ remediation of groundwater nitrate pollution, yet because of the low redox state they induce on the subsurface environment there is an inherent risk they can promote pollution-swapping phenomena. We evaluated pollution-swapping phenomena associated with the first two operational years of a woodchip denitrification wall that is being trialled in a fast-flowing shallow gravel aquifer of quartzo-feldspathic mineralogy. Following burial of woodchip below the water table there was immediate export of dissolved organic carbon (DOC), phosphorus and ammonium into the groundwater. Under the low redox state sustained by labile DOC, the wall initially provided 100% nitrate removal at the expense of acute and localised pollution that occurred in the form of a plume of dissolved iron, manganese and arsenic that were mobilised from the aquifer sediments, in conjunction with methane gas emission. Within one year however, the reactivity of the woodchip wall subsided to support a steady state condition in which nitrate reduction was the terminal electron acceptor process with no measurable methane emission. Having initially functioned as a sink for the potent greenhouse gas nitrous oxide (N₂O), evidence is that the woodchip wall is now exporting N₂O, albeit at rates less than those associated with productive agricultural land.

Investigating the role of organic carbon amendments and microbial denitrification gene abundance in nitrogen removal from experimental agricultural drainage ditches with low-grade weirs

Low-grade weirs placed within agricultural drainage ditches in the Lower Mississippi Alluvial Valley can be used as a management practice to enhance nitrogen removal. The addition of organic carbon amendments within ditches that contain weirs could further increase nitrogen removal. Through repeated trials, changes in NO 3 - -N concentration between inflow and outflow were variable in the ditch without weirs, while only decreases in concentration were observed in ditches with weirs. Significant differences in NO 3 - -N concentrations were observed between treatments, with greater removal of NO 3 - -N observed in dissolved organic carbon treatments compared to control and particulate organic carbon treatments. At medium- and high-flow rates, respectively, dissolved organic carbon treatments resulted in greater NO 3 - -N concentration decreases of 31.6% and 27.1% compared to 19% and 11.6% in particulate organic carbon treatments and 18.6% and 17.2% in control treatments. Significant effects of weirs and sampling date on nirS, nirK, nosZ, and 16S rRNA gene abundances were observed. Observed increases in NO 3 - -N removal with organic carbon amendments, provides support for continued investigation on improving the efficacy of organic carbon amendments as a best management practice for NO 3 - -N removal in agricultural drainage ditches. PRACTITIONER POINTS: Dissolved organic carbon amendments increased nitrate-nitrogen removal. Only decreases in nitrate-nitrogen concentration were observed in ditches with weirs. Increasing flow rate did not affect nitrate-nitrogen removal. Abundance of denitrification-performing microbes likely did not affect N removal. Lack of anaerobic soil conditions and short residence time reduced nitrate-N removal.

Enhanced nitrate removal from surface water in a denitrifying woodchip bioreactor with a heterotrophic nitrifying and aerobic denitrifying fungus

A heterotrophic nitrifying and aerobic denitrifying fungus was isolated from lake water and identified as Penicillium tropicum strain IS0293. The strain exhibited efficient heterotrophic nitrification-aerobic denitrification ability and could utilize ammonium, nitrite and nitrate as a sole nitrogen source. Batch tests demonstrated that strain IS0293 can remove nitrate using variety of organic carbon compounds as carbon sources. The effect of woodchip leachate collected at different degradation times on denitrification performance of the strain was also investigated. Furthermore, two denitrifying woodchip bioreactors were constructed to assess the bioaugmention of strain IS0293 for nitrate removal from surface water. Results demonstrated that the incubation of strain IS0293 enhanced the nitrate removal efficiency of the bioreactor. In addition, the average effluent TOC content of the bioaugmention bioreactor was 38.22% lower than the control bioreactor. This study would be valuable to develop an effective technology for nitrate-laden surface water under aerobic conditions.

Nutrient and pesticide remediation using a two-stage bioreactor-adsorptive system under two hydraulic retention times

Nutrients and pesticides in agricultural runoff contribute to the degradation of water resources. Nitrates and phosphates can be remediated through the use of treatment systems such as woodchip bioreactors and adsorbent aggregate filters; however, concerns remain over potential effects of pesticides on nutrient removal efficiency in these systems. To test this, we designed laboratory-scale woodchip bioreactors equipped with secondary adsorbent aggregate filters and investigated the capacity of these systems to remediate nutrients when operated under two hydraulic retention times (HRT) and in the presence of commonly used pesticides. The woodchip bioreactors effectively removed over 99% of nitrate per day when operated under a 72 h hydraulic retention time, with the secondary expanded shale aggregate filters consistently reducing phosphate concentrations by 80-87%. Treatment efficacy of both systems was maintained in the presence of the insecticide chlorpyrifos. Reducing HRT in the bioreactors to 21 min decreased nitrate removal efficiency; however, the insecticides bifenthrin, chlorpyrifos, and the herbicide oxyfluorfen were reduced by 76%, 63%, and 31%, respectively. Cultivation approaches led to the isolation of 45 different species from the woodchip bioreactors operated under a 21 min HRT, with Bacillus species being the most prevalent throughout the treatment. By contrast, pesticide application decreased the number and diversity of Bacillus isolates and enriched for Pseudomonas and Exiguobacterium species. Woodchip bioreactors and adsorbent aggregate filters provide effective treatment platforms to remediate agrochemicals, where they maintain treatment efficacy in the presence of pesticides and can be modulated through HRT management to achieve environmental and operational water quality goals.

Nitrous oxide and methane production from denitrifying woodchip bioreactors at three hydraulic residence times

Denitrifying bioreactors remove nitrate (NO₃^-) from agricultural drainage and are slated to be an integral part of nitrogen reduction strategies in the Mississippi River Basin. However, incomplete denitrification can result in nitrous oxide (N₂O) production and anaerobic conditions within bioreactors may be conducive to methane (CH₄) production via methanogenesis. Greenhouse gas production has the potential to trade excess NO₃^- in surface water with excess greenhouses gases in the atmosphere. Our study examined N₂O and CH₄ production from pilot scale (6.38 m³) bioreactors across three hydraulic residence times (HRTs), 2, 8, and 16 h. Production was measured from both the surface of the bioreactors and dissolved in the bioreactor effluent. Nitrous oxide and CH₄ was produced across all HRTs, with the majority dissolved in the effluent. Nitrous oxide production was significantly greater (P < 0.05) from 2 h HRTs (478.43 mg N₂O m^-3 day^-1) than from 8 (29.95 mg N₂O m^-3 day^-1) and 16 (36.61 mg N₂O m^-3 day^-1) hour HRTs. Methane production was significantly less (P < 0.05) from 2 h HRTs (0.51 g C m³ day^-1) compared to 8 (1.50 g C m³ day^-1) and 16 (1.69 g C m³ day^-1) hour HRTs. The 2 h HRTs had significantly greater (P = 0.05) impacts to climate change compared to 8 and 16 h HRTs. Results from this study suggest managing HRTs between 6 and 8 h in field bioreactors could minimize total greenhouse gas production and maximize NO₃^- removal.

Quantifying the contribution of tile drainage to basin-scale water yield using analytical and numerical models

The Des Moines Lobe (DML) of north-central Iowa has been artificially drained by subsurface drains and surface ditches to provide some of the most productive agricultural land in the world. Herein we report on the use of end-member mixing analysis (EMMA) models and the numerical model Soil and Water Assessment Tool (SWAT) to quantify the contribution of tile drainage to basin-scale water yields at various scales within the 2370 km² Boone River watershed (BRW), a subbasin within the Des Moines River watershed. EMMA and SWAT methods suggested that tile drainage provided approximately 46 to 54% of annual discharge in the Boone River and during the March to June period, accounted for a majority of flow in the river. In the BRW subbasin of Lyons Creek, approximately 66% of the annual flow was sourced from tile drainage. Within the DML region, tile drainage contributes to basin-scale water yields at scales ranging from 40 to 16,000 km², with downstream effects diminishing with increasing watershed size. Developing a better understanding of water sources contributing to river discharge is needed if mitigation and control strategies are going to be successfully targeted to reduce downstream nutrient export.

Effects of mercury addition on microbial community composition and nitrate removal inside permeable reactive barriers

Permeable reactive barriers (PRBs) remove nitrogen from groundwater by enhancing microbial denitrification. The PRBs consist of woodchips that provide carbon for denitrifiers, but these woodchips also support other anaerobic microbes, including sulfate-reducing bacteria. Some of these anaerobes have the ability to methylate inorganic mercury present in groundwater. Methylmercury is hazardous to human health, so it is essential to understand whether PRBs promote mercury methylation. We examined microbial communities and geochemistry in fresh water and sulfate-enriched PRB flow-through columns by spiking replicates of both treatments with mercuric chloride. We hypothesized that mercury addition could alter bacterial community composition to favor higher abundances of genera containing known methylating taxa and that the sulfate-rich columns would produce more methylmercury after mercury addition, due mainly to an increase in abundance of sulfate reducing bacteria (SRB). However, methylmercury output at the end of the experiment was not different from output at the beginning, due in part to coupled Hg methylation and demethylation. There was a transient reduction in nitrate removal after mercury addition in the sulfate enriched columns, but nitrate removal returned to initial rates after two weeks, demonstrating resilience of the denitrifying community. Since methylmercury output did not increase and nitrate removal was not permanently affected, PRBs could be a low cost approach to combat eutrophication.

Carbon Dosing Increases Nitrate Removal Rates in Denitrifying Bioreactors at Low-Temperature High-Flow Conditions

Nitrogen losses from croplands contribute to impairment of water bodies. This laboratory experiment evaluated various C sources for use in a denitrifying bioreactor, a conservation practice designed to reduce N losses. The nitrate removal efficiency of candidate treatments (corn cobs [CC], corn cobs with modified coconut coir [CC+MC], corn cobs with modified coconut coir and modified macadamia shell biochar [CC+MC+MBC], wood chips [WC], wood chips with hardwood biochar [WC+BC], and wood chips with continuous sodium acetate addition [WC+A]) were tested with up-flow direction. Effluent was sampled after a repeated weekly flow regime with hydraulic residence times of 1.5, 8, 12, and 24 h. Column temperatures were 15°C for 14 wk (warm), 5°C for 13 wk (cold), and again 15°C for 7 wk (rewarm). Cumulative nitrate N load reduction was greatest for WC+A (80, 80, and 97% during the warm, cold, and rewarm runs, respectively). Corn cob treatments (CC, CC+MC, and CC+MC+MBC) had the second greatest cumulative load reductions for all three temperature experiments, and WC and WC+BC had the lowest performance under these conditions. The nitrate removal rate was optimum at the 1.5-h hydraulic residence time for the WC+A treatment: 43, 30, and 121 g N m d for the warm, cold, and rewarm runs, respectively. Furthermore, acetate addition greatly improved wood chip performance and could be used to enhance nitrate N removal under the cold and high-flow-rate conditions of springtime drainage for the north-central United States.

Evaluation of fly ash pellets for phosphorus removal in a laboratory scale denitrifying bioreactor

Nitrate and orthophosphate from agricultural activities contribute significantly to nutrient loading in surface water bodies around the world. This study evaluated the efficacy of woodchips and fly ash pellets in tandem to remove nitrate and orthophosphate from simulated agricultural runoff in flow-through tests. The fly ash pellets had previously been developed specifically for orthophosphate removal for this type of application, and the sorption bench testing showed a good promise for flow-through testing. The lab-scale horizontal-flow bioreactor used in this study consisted of an upstream column filled with woodchips followed by a downstream column filled with fly ash pellets (3 and 1 m lengths, respectively; both 0.15 m diameter). Using influent concentrations of 12 mg/L nitrate and 5 mg/L orthophosphate, the woodchip bioreactor section was able to remove 49-85% of the nitrate concentration at three hydraulic retention times ranging from 0.67 to 4.0 h. The nitrate removal rate for woodchips ranged from 40 to 49 g N/m³/d. Higher hydraulic retention times (i.e., smaller flow rates) corresponded with greater nitrate load reduction. The fly ash pellets showed relatively stable removal efficiency of 68-75% across all retention times. Total orthophosphate adsorption by the pellets was 0.059-0.114 mg P/g which was far less than the saturated capacity (1.69 mg/g; based on previous work). The fly ash pellets also removed some nitrate and the woodchips also removed some orthophosphate, but these reductions were not significant. Overall, woodchip denitrification followed by fly ash pellet P-sorption can be an effective treatment technology for nitrate and phosphate removal in subsurface drainage.

Evaluation of Mechanistic Models for Nitrate Removal in Woodchip Bioreactors

Woodchip bioreactors (WBRs) are increasingly being applied to remove nitrate from runoff. In this study, replicate columns with aged woodchips were subjected to a range of measured flow rates and influent nitrate concentrations with an artificial stormwater matrix. Dissolved oxygen (DO), nitrate, and dissolved organic carbon (DOC) were measured along the length of the columns. A multispecies reactive transport model with Michaelis-Menten kinetics was developed to explain the concentration profiles of DO, nitrate, and DOC. Four additional models were developed based on simplifying assumptions, and all five models were tested for their ability to predict nitrate concentrations in the experimental columns. Global sensitivity analysis and constrained optimization determined the set of parameters that minimized the root-mean-squared error (RMSE) between the model and the experimental data. A k-fold validation test revealed no statistical difference in RMSE for predicting nitrate concentrations between a zero-order model and the other multispecies reactive transport models tested. Additionally, the multispecies reactive transport models demonstrated no significant differences in predicting DO and DOC concentrations. These results suggest that denitrification in an aged woodchip bioreactor at constant temperature can effectively be modeled using zero-order kinetics when nitrate concentrations are >2 mg-N L^-1. A multispecies model may be used if predicting DOC or DO concentrations is desired.

High Removal of Effluent-borne Nitrogen with Multiple External Electron Donors in the Engineered Drainfield of an Advanced Septic System

Septic systems can be a major source of nitrogen (N) in shallow groundwater. We designed an in situ engineered drainfield with aerobic-anaerobic (sand-woodchips) and anaerobic (elemental sulfur-oyster shell) media to remove N in the vadose zone and reduce N transport to groundwater. Effluent was dispersed on top of the engineered drainfield (3.72 m infiltrative surface) and then infiltrated through the aerobic-anaerobic and anaerobic media before reaching natural soil. Water samples were collected over 64 sampling events (May 2012-December 2013) from three parts of the drainfield: (i) a suction cup lysimeter installed at the sand-woodchips interface, (ii) a pipe after effluent passed through the aerobic-anaerobic media, and (iii) a tank containing anaerobic media. In the effluent, most of the total N (66 mg L) was present as NH-N (88.8%), whereas at the sand-woodchips interface the dominant N form was NO-N (31 mg L; 85% of total N). As the effluent passed through the aerobic-anaerobic media in the drainfield, heterotrophic denitrification reduced NO-N to 5.4 mg L. In the tank containing anaerobic media, autotrophic denitrification, facilitated by elemental sulfur, further reduced NO-N to 1 mg L. Overall, 90% of total added N was removed as the effluent passed through the aerobic-anaerobic and anaerobic media within the engineered drainfield. We conclude that the use of multiple electron donors from external media (sand-woodchips and elemental sulfur-oyster shell) was effective at removing N in the engineered drainfield and will reduce the risk of groundwater N contamination from septic systems in areas with shallow groundwater.

Evaluation of Phosphorus Filter Media for an Inline Subsurface Drainage Treatment System

Subsurface drainage from agricultural land has been identified as a contributor of both N and P into surface waters, leading to water quality degradation and eutrophication. This study evaluates the ability of P sorption media (PSM; expanded shale, expanded clay, furnace slag, and natural soil) to sorb P in both batch and column tests. Batch sorption tests estimated sorption of 3.4, 1.2, and 0.5 g P kg for expanded shale, expanded clay, and natural soil, respectively. Furnace slag sorption was evaluated for fine (FS), small (FS), and large (FS) particle sizes, with estimated sorption of 6.8, 5.1, and 3.8 g P kg, respectively. Phosphorus removal for the three furnace slag particle sizes and natural soil were tested in flow-through columns operated at residence times of 50, 17, and 7 s. A decrease in residence time reduced P removal in all columns evaluated. Following all trials, the average P removal from influent was 50% for FS, followed by 27% for FS (furnace slag-coated pea gravel), 22% for FS, and 6% for sandy loam-coated pea gravel. The data from this study provides crucial information for developing and sizing an inline tile drainage treatment system to remove P from tile drainage outlets before reaching surface waters.

Nitrate and phosphate removal from agricultural subsurface drainage using laboratory woodchip bioreactors and recycled steel byproduct filters

Woodchip bioreactors have been increasingly used as an edge-of-field treatment technology to reduce the nitrate loadings to surface waters from agricultural subsurface drainage. Recent studies have shown that subsurface drainage can also contribute substantially to the loss of phosphate from agricultural soils. The objective of this study was to investigate nitrate and phosphate removal in subsurface drainage using laboratory woodchip bioreactors and recycled steel byproduct filters. The woodchip bioreactor demonstrated average nitrate removal efficiencies of 53.5-100% and removal rates of 10.1-21.6 g N/m(3)/d for an influent concentration of 20 mg N/L and hydraulic retention times (HRTs) of 6-24 h. When the influent nitrate concentration increased to 50 mg N/L, the bioreactor nitrate removal efficiency and rate averaged 75% and 18.9 g N/m(3)/d at an HRT of 24 h. Nitrate removal by the woodchips followed zero-order kinetics with rate constants of 1.42-1.80 mg N/L/h when nitrate was non-limiting. The steel byproduct filter effectively removed phosphate in the bioreactor effluent and the total phosphate adsorption capacity was 3.70 mg P/g under continuous flow conditions. Nitrite accumulation occurred in the woodchip bioreactor and the effluent nitrite concentrations increased with decreasing HRTs and increasing influent nitrate concentrations. The steel byproduct filter efficiently reduced the level of nitrite in the bioreactor effluent. Overall, the results of this study suggest that woodchip denitrification followed by steel byproduct filtration is an effective treatment technology for nitrate and phosphate removal in subsurface drainage.

Soil amendment using poplar woodchips to enhance the treatment of wastewater-originated nutrients

Vegetation filters, a nature based wastewater regeneration technology, have been reported as a feasible solution for small municipalities and scattered populations with limited access to sewage networks. However even when such a treatment is properly planned, the leaching of contaminants through the unsaturated zone may occur. The amendment of soil with a readily-labile source of carbon is supposed to ameliorate the removal of contaminants by stimulating microbial activity and enhancing sorption processes. In this study, lab-scale leaching column experiments were carried out to explore if the addition of woodchips to the soil could be a feasible strategy to be integrated in a vegetation filter. Two different types of arrangement of soil and woodchips layers were tested. The soil was collected from an operating vegetation filter treating wastewater of an office building characterised by a high nutrient load. Daily pulse of synthetic wastewater were applied into the columns and effluent samples were collected and analyzed for major ions, total nitrogen (NT), total phosphorous (PT) and chemical oxygen demand (COD). By the end of the experiment, NT, NO3-N and PT soil contents were also measured. Results indicate that amendments with woodchips enhance the elimination of wastewater-originated contaminants. NT removal in the columns with woodchips reaches a value of 99.4%. The main processes responsible for this elimination are NH4-N sorption and nitrification/denitrification. This latter fostered by the reduced redox conditions due to the enhanced microbial activity. High removal of PT (99%) is achieved independently of the woodchips presence due to retention and/or precipitation phenomena. The COD removal efficiency is not affected by the presence of the woodchips. The leaching of organic carbon occurs only during the experimental start-up period.

Simulating Woodchip Bioreactor Performance Using a Dual-Porosity Model

There is a general understanding in the scientific community as to how denitrifying bioreactors operate, but we lack a quantitative understanding of the details of the denitrification process acting within them and comprehensive models for simulating their performance. We hypothesized that nitrate transport through woodchip bioreactors would be best described by a dual-porosity transport model where the bioreactor water is divided into a mobile domain (i.e., the water between the woodchips where it is free to flow and solute movement is by advection and dispersion) and an immobile domain of water (i.e., the water mostly within the woodchips that is stagnant and where solute movement is by diffusion alone). We calibrated the dual-porosity model contained in the HYDRUS model for a woodchip bioreactor using the results of a Br breakthrough experiment where we treated Br as a conservative nonadsorbing tracer. We then used the resulting model parameters to describe 2 yr of NO transport and denitrification within a bioreactor supplied by tile drainage. The only model parameters fitted to the NO data were either the zero- or first-order denitrification rate and its temperature dependence. The bioreactor denitrified 2.23 kg N (38%) of the NO entering it in 2013 and 3.73 kg N (49%) of the NO that entered it in 2014. The dual-porosity model fit the NO data very well, with fitted zero-order reaction rates of 8.7 and 6.8 mg N L d in 2013 and 2014, respectively, and corresponding first-order reaction rates of 0.99 and 1.02 d. For the 2-yr data set, both reaction rate models fit the data equally well. Consistent model parameters fitted for the 2 yr indicated that the model used was robust and a promising approach for modeling fate and transport of NO in woodchip bioreactors.

Use Alkalinity Monitoring to Optimize Bioreactor Performance

In recent years, the agricultural community has reduced flow of nitrogen from farmed landscapes to stream networks through the use of woodchip denitrification bioreactors. Although deployment of this practice is becoming more common to treat high-nitrate water from agricultural drainage pipes, information about bioreactor management strategies is sparse. This study focuses on the use of water monitoring, and especially the use of alkalinity monitoring, in five Iowa woodchip bioreactors to provide insights into and to help manage bioreactor chemistry in ways that will produce desirable outcomes. Results reported here for the five bioreactors show average annual nitrate load reductions between 50 and 80%, which is acceptable according to established practice standards. Alkalinity data, however, imply that nitrous oxide formation may have regularly occurred in at least three of the bioreactors that are considered to be closed systems. Nitrous oxide measurements of influent and effluent water provide evidence that alkalinity may be an important indicator of bioreactor performance. Bioreactor chemistry can be managed by manipulation of water throughput in ways that produce adequate nitrate removal while preventing undesirable side effects. We conclude that (i) water should be retained for longer periods of time in bioreactors where nitrous oxide formation is indicated, (ii) measuring only nitrate and sulfate concentrations is insufficient for proper bioreactor operation, and (iii) alkalinity monitoring should be implemented into protocols for bioreactor management.

The role of C:N:P stoichiometry in affecting denitrification in sediments from agricultural surface and tile-water wetlands

Nutrient stoichiometry within a wetland is affected by the surrounding land use, and may play a significant role in the removal of nitrate (NO3-N). Tile-drained, agricultural watersheds experience high seasonal inputs of NO3-N, but low phosphorus (PO4-P) and dissolved organic carbon (DOC) loads relative to surface water dominated systems. This difference may present stoichiometric conditions that limit denitrification within receiving waterways. We investigated how C:N:P ratios affected denitrification rates of sediments from tile-drained mitigation wetlands incubated for: 0, 5, 10, and 20 days. We then tested whether denitrification rates of sediments from surface-water and tile-drained wetlands responded differently to C:N ratios of 2:1 versus 4:1. Ratios of C:N:P (P < 0.05) and incubation length (P < 0.05) had a significant effect on denitrification in tile-drained wetland sediments. Carbon limitation of denitrification became evident at elevated NO3-N concentrations (20 mg L(-1)). Denitrification measured from tile water and surface water wetland sediments increased significantly (P < 0.05) at the 2:1 and 4:1 C:N treatments. The results from both experiments suggest wetland sediments provide a limiting pool of labile DOC to maintain prolonged NO3-N removal. Also, DOC limitation became more evident at elevated NO3-N concentrations (20 mg L(-1)). Irrespective of NO3-N concentrations, P did not limit denitrification rates. In addition to wetting period, residence time, and maintenance of anaerobic conditions, the availability of labile DOC is playing an important limiting role in sediment denitrification within mitigation wetlands.

Efficiency and detrimental side effects of denitrifying bioreactors for nitrate reduction in drainage water

A laboratory column experiment was conducted to test the efficiency of denitrifying bioreactors for the nitrate (NO3-N) removal in drainage waters at different flow rates and after desiccation. In addition, we investigated detrimental side effects in terms of the release of nitrite (NO2-N), ammonium (NH4-N), phosphate (PO4-P), dissolved organic carbon (DOC), methane (CH4), and dinitrogen oxide (N2O). The NO3-N removal efficiency decreased with increasing NO3-N concentrations, increasing flow rates, and after desiccation. Bioreactors with purely organic fillings showed higher NO3-N removal rates (42.6-55.7 g NO3-N m(-3) day(-1)) than those with organic and inorganic fillings (6.5-21.4 g NO3-N m(-3) day(-1)). The release of NO2-N and DOC was considerable and resulted in concentrations of up to 800 μg NO2-N L(-1)and 25 mg DOC L(-1) in the effluent water. N2O concentrations increased by 4.0 to 15.3 μg N2O-N L(-1) between the influent and the effluent, while CH4 production rates were low. Our study confirms the high potential of denitrifying bioreactors to mitigate NO3-N pollution in drainage waters, but highlights also the potential risks for the environment.

Characterizing the Performance of Denitrifying Bioreactors during Simulated Subsurface Drainage Events

The need to mitigate nitrate export from corn and soybean fields with subsurface (tile) drainage systems, a major environmental issue in the midwestern United States, has made the efficacy of field-edge, subsurface bioreactors an active subject of research. This study of three such bioreactors located on the University of Illinois South Farms during their first 6 mo of operation (July-Dec. 2012) focused on the interactions of seasonal temperature changes and hydraulic retention times (HRTs), which were subject to experimental manipulation. Changes in nitrate, phosphate, oxygen, and dissolved organic carbon were monitored in influent and effluent to assess the benefits and the potential harmful effects of bioreactors for nearby aquatic ecosystems. On average, bioreactors reduced nitrate loads by 63%, with minimum and maximum reductions of 20 and 98% at low and high HRTs, respectively. The removal rate per unit reactor volume averaged 11.6 g NO-N m d (range, 5-30 g NO-N m d). Multiple regression models with exponential dependencies on influent water temperature and on HRT explained 73% of the variance in NO-N load reduction and 43% of the variance in its removal rate. Although concentrations of dissolved reactive phosphorus and dissolved organic carbon in the bioreactor effluent increased relative to the influent by an order of magnitude during initial tests, within 1 mo of operation they stabilized at nearly equal values.

Permeable Reactive Barriers Designed To Mitigate Eutrophication Alter Bacterial Community Composition and Aquifer Redox Conditions

Permeable reactive barriers (PRBs) consist of a labile carbon source that is positioned to intercept nitrate-laden groundwater to prevent eutrophication. Decomposition of carbon in the PRB drives groundwater anoxic, fostering microbial denitrification. Such PRBs are an ideal habitat to examine microbial community structure under high-nitrate, carbon-replete conditions in coastal aquifers. We examined a PRB installed at the Waquoit Bay National Estuarine Research Reserve in Falmouth, MA. Groundwater within and below the PRB was depleted in oxygen compared to groundwater at sites upgradient and at adjacent reference sites. Nitrate concentrations declined from a high of 25 μM upgradient and adjacent to the barrier to <0.1 μM within the PRB. We analyzed the total and active bacterial communities filtered from groundwater flowing through the PRB using amplicons of 16S rRNA and of the 16S rRNA genes. Analysis of the 16S rRNA genes collected from the PRB showed that the total bacterial community had high relative abundances of bacteria thought to have alternative metabolisms, such as fermentation, including candidate phyla OD1, OP3, TM7, and GN02. In contrast, the active bacteria had lower abundances of many of these bacteria, suggesting that the bacterial taxa that differentiate the PRB groundwater community were not actively growing. Among the environmental variables analyzed, dissolved oxygen concentration explained the largest proportion of total community structure. There was, however, no significant correlation between measured environmental parameters and the active microbial community, suggesting that controls on the active portion may differ from the community as a whole.

Modeling nitrate removal in a denitrification bed

Denitrification beds are promoted to reduce nitrate load in agricultural subsurface drainage water to alleviate the adverse environmental effects associated with nitrate pollution of surface water. In this system, drainage water flows through a trench filled with a carbon media where nitrate is transformed into nitrogen gas under anaerobic conditions. The main objectives of this study were to model a denitrification bed treating drainage water and evaluate its adverse greenhouse gas emissions. Field experiments were conducted at an existing denitrification bed. Evaluations showed very low greenhouse gas emissions (mean N2O emission of 0.12 μg N m(-2) min(-1)) from the denitrification bed surface. Field experiments indicated that nitrate removal rate was described by Michaelis-Menten kinetics with the Michaelis-Menten constant of 7.2 mg N L(-1). We developed a novel denitrification bed model based on the governing equations for water flow and nitrate removal kinetics. The model evaluation statistics showed satisfactory prediction of bed outflow nitrate concentration during subsurface drainage flow. The model can be used to design denitrification beds with efficient nitrate removal which in turn leads to enhanced drainage water quality.

Agro-hydrologic landscapes in the Upper Mississippi and Ohio River basins

A critical part of increasing conservation effectiveness is targeting the "right practice" to the "right place" where it can intercept pollutant flowpaths. Conceptually, these flowpaths can be inferred from soil and slope characteristics, and in this study, we developed an agro-hydrologic classification to identify N and P loss pathways and priority conservation practices in small watersheds in the U.S. Midwest. We developed a GIS framework to classify 11,010 small watersheds in the Upper Mississippi and Ohio River basins based on soil permeability and slope characteristics of agricultural cropland areas in each watershed. The amount of cropland in any given watershed varied from <10 to >60 %. Cropland areas were classified into five main categories, with slope classes of <2, 2-5, and >5 %, and soil drainage classes of poorly and well drained. Watersheds in the Upper Mississippi River basin (UMRB) were dominated by cropland areas in low slopes and poorly drained soils, whereas less-intensively cropped watersheds in Wisconsin and Minnesota (in the UMRB) and throughout the Ohio River basin were overwhelmingly well drained. Hydrologic differences in cropped systems indicate that a one-size-fits-all approach to conservation selection will not work. Consulting the classification scheme proposed herein may be an appropriate first-step in identifying those conservation practices that might be most appropriate for small watersheds in the basin.

Low transient storage and uptake efficiencies in seven agricultural streams: implications for nutrient demand

We used mass load budgets, transient storage modeling, and nutrient spiraling metrics to characterize nitrate (NO), ammonium (NH), and inorganic phosphorus (SRP) demand in seven agricultural streams across the United States and to identify in-stream services that may control these conditions. Retention of one or all nutrients was observed in all but one stream, but demand for all nutrients was low relative to the mass in transport. Transient storage metrics (/, , , and ) correlated with NO retention but not NH or SRP retention, suggesting in-stream services associated with transient storage and stream water residence time could influence reach-scale NO demand. However, because the fraction of median reach-scale travel time due to transient storage () was ≤1.2% across the sites, only a relatively small demand for NO could be generated by transient storage. In contrast, net uptake of nutrients from the water column calculated from nutrient spiraling metrics were not significant at any site because uptake lengths calculated from background nutrient concentrations were statistically insignificant and therefore much longer than the study reaches. These results suggest that low transient storage coupled with high surface water NO inputs have resulted in uptake efficiencies that are not sufficient to offset groundwater inputs of N. Nutrient retention has been linked to physical and hydrogeologic elements that drive flow through transient storage areas where residence time and biotic contact are maximized; however, our findings indicate that similar mechanisms are unable to generate a significant nutrient demand in these streams relative to the loads.

Reconnecting tile drainage to riparian buffer hydrology for enhanced nitrate removal

Riparian buffers are a proven practice for removing NO from overland flow and shallow groundwater. However, in landscapes with artificial subsurface (tile) drainage, most of the subsurface flow leaving fields is passed through the buffers in drainage pipes, leaving little opportunity for NO removal. We investigated the feasibility of re-routing a fraction of field tile drainage as subsurface flow through a riparian buffer for increasing NO removal. We intercepted an existing field tile outlet draining a 10.1-ha area of a row-cropped field in central Iowa and re-routed a fraction of the discharge as subsurface flow along 335 m of an existing riparian buffer. Tile drainage from the field was infiltrated through a perforated pipe installed 75 cm below the surface by maintaining a constant head in the pipe at a control box installed in-line with the existing field outlet. During 2 yr, >18,000 m (55%) of the total flow from the tile outlet was redirected as infiltration within the riparian buffer. The redirected water seeped through the 60-m-wide buffer, raising the water table approximately 35 cm. The redirected tile flow contained 228 kg of NO. On the basis of the strong decrease in NO concentrations within the shallow groundwater across the buffer, we hypothesize that the NO did not enter the stream but was removed within the buffer by plant uptake, microbial immobilization, or denitrification. Redirecting tile drainage as subsurface flow through a riparian buffer increased its NO removal benefit and is a promising management practice to improve surface water quality within tile-drained landscapes.

Spatial variation in the bacterial and denitrifying bacterial community in a biofilter treating subsurface agricultural drainage

Denitrifying biofilters can remove agricultural nitrates from subsurface drainage, reducing nitrate pollution that contributes to coastal hypoxic zones. The performance and reliability of natural and engineered systems dependent upon microbially mediated processes, such as the denitrifying biofilters, can be affected by the spatial structure of their microbial communities. Furthermore, our understanding of the relationship between microbial community composition and function is influenced by the spatial distribution of samples.In this study we characterized the spatial structure of bacterial communities in a denitrifying biofilter in central Illinois. Bacterial communities were assessed using automated ribosomal intergenic spacer analysis for bacteria and terminal restriction fragment length polymorphism of nosZ for denitrifying bacteria.Non-metric multidimensional scaling and analysis of similarity (ANOSIM) analyses indicated that bacteria showed statistically significant spatial structure by depth and transect,while denitrifying bacteria did not exhibit significant spatial structure. For determination of spatial patterns, we developed a package of automated functions for the R statistical environment that allows directional analysis of microbial community composition data using either ANOSIM or Mantel statistics.Applying this package to the biofilter data, the flow path correlation range for the bacterial community was 6.4 m at the shallower, periodically in undated depth and 10.7 m at the deeper, continually submerged depth. These spatial structures suggest a strong influence of hydrology on the microbial community composition in these denitrifying biofilters. Understanding such spatial structure can also guide optimal sample collection strategies for microbial community analyses.

Evaluation of a denitrification wall to reduce surface water nitrogen loads

Denitrification walls have significantly reduced nitrogen concentrations in groundwater for at least 15 yr. This has spurred interest in developing methods to efficiently increase capture volume to reduce N loads in larger watersheds. The objective of this study was to maximize treatment volume by locating a wall where a large groundwatershed was funneled toward seepage slope headwaters. Nitrogen concentration and load were measured before and after wall installation in paired treatment and control streams. Beginning 2 d after installation, nitrogen concentration in the treatment stream declined from 6.7 ± 1.2 to 3.9 ± 0.78 mg L and total N loading rate declined by 65% (391 kg yr) with no corresponding decline in the control watershed. This wall, which only comprised 10 to 11% of the edge of field area that contributed to the treatment watershed, treated approximately 60% of the stream discharge, which confirmed the targeted approach. The total load reduction measured in the stream 155 m downstream from the wall (340 kg yr) was higher than that found in another study that measured load reductions in groundwater wells immediately around the wall (228 kg yr). This indicated the possibility of an extended impact on denitrification from carbon exported beyond the wall. This extended impact was inauspiciously confirmed when oxygen levels at the stream headwaters temporarily declined for 50 d. This research indicates that targeting walls adjacent to streams can effectively reduce N loading in receiving waters, although with a potentially short-term impact on water quality.

Evaluation of simulated strategies for reducing nitrate-nitrogen losses through subsurface drainage systems

The nitrates (NO(3)-N) lost through subsurface drainage in the Midwest often exceed concentrations that cause deleterious effects on the receiving streams and lead to hypoxic conditions in the northern Gulf of Mexico. The use of drainage and water quality models along with observed data analysis may provide new insight into the water and nutrient balance in drained agricultural lands and enable evaluation of appropriate measures for reducing NO(3)-N losses. DRAINMOD-NII, a carbon (C) and nitrogen (N) simulation model, was field tested for the high organic matter Drummer soil in Indiana and used to predict the effects of fertilizer application rate and drainage water management (DWM) on NO-N losses through subsurface drainage. The model was calibrated and validated for continuous corn (Zea mays L.) (CC) and corn-soybean [Glycine max (L.) Merr.] (CS) rotation treatments separately using 7 yr of drain flow and NO(3)-N concentration data. Among the treatments, the Nash-Sutcliffe efficiency of the monthly NO(3)-N loss predictions ranged from 0.30 to 0.86, and the percent error varied from -19 to 9%. The medians of the observed and predicted monthly NO(3)-N losses were not significantly different. When the fertilizer application rate was reduced ~20%, the predicted NO(3)-N losses in drain flow from the CC treatments was reduced 17% (95% confidence interval [CI], 11-25), while losses from the CS treatment were reduced by 10% (95% CI, 1-15). With DWM, the predicted average annual drain flow was reduced by about 56% (95% CI, 49-67), while the average annual NO(3)-N losses through drain flow were reduced by about 46% (95% CI, 32-57) for both tested crop rotations. However, the simulated NO(3)-N losses in surface runoff increased by about 3 to 4 kg ha(-1) with DWM. For the simulated conditions at the study site, implementing DWM along with reduced fertilizer application rates would be the best strategy to achieve the highest NO(3)-N loss reductions to surface water. The suggested best strategies would reduce the NO(3)-N losses to surface water by 38% (95% CI, 29-46) for the CC treatments and by 32% (95% CI, 23-40) for the CS treatments.

Dissipation of atrazine, enrofloxacin, and sulfamethazine in wood chip bioreactors and impact on denitrification

Wood chip bioreactors are receiving increasing attention as a means of reducing nitrate in subsurface tile drainage systems. Agrochemicals in tile drainage water entering wood chip bioreactors can be retained or degraded and may affect denitrification. The degradation of 5 mg L atrazine, enrofloxacin, and sulfamethazine under denitrifying conditions in wood chips from an in situ reactor was determined. The impact of these chemicals on denitrifying microorganisms was assessed using the denitrification potential assay, most probable number (MPN), and quantitative polymerase chain reaction targeting the gene of the denitrifiers. Initial half-lives for these chemicals in the aqueous phase were 0.98 d for atrazine, 0.17 d for enrofloxacin, and 6.2 d for sulfamethazine. Similar rates of disappearance in autoclaved and nonautoclaved wood chip solutions during the first 48 h suggested sorption was the dominant mechanism. The presence of atrazine did not impair denitrification potential, the MPN, or the copy number. The denitrifier MPN and copy number in sulfamethazine- and enrofloxacin-treated microcosms were less than the control within the first 5 d after chemical addition, whereas the denitrification potentials were not affected. However, after 45 d the denitrification rate, MPN and gene copy numbers for sulfamethazine and enrofloxacin were similar to that of the no-chemical control, indicating that acclimation of the denitrifier population to the antibiotic or reduced bioavailability over time allowed recovery of the denitrifier population.

Nitrate controls methyl mercury production in a streambed bioreactor

Organic carbon bioreactors provide low-cost, passive treatment of a variety of environmental contaminants but can have undesirable side effects in some cases. This study examines the production of methyl mercury (MeHg) in a streambed bioreactor consisting of 40 m³ of wood chips and designed to treat nitrate (NO₃) in an agricultural drainage ditch in southern Ontario (Avon site). The reactor provides 30 to 100% removal of NO₃-N concentrations of 0.6 to 4.4 mg L(-1), but sulfate (SO₄(2-)) reducing conditions develop when NO₃ removal is complete. Sulfate reducing conditions are known to stimulation the production of MeHg in natural wetlands. Over one seasonal cycle, effluent MeHg ranged from 0.01 to 0.76 ng L(-1) and total Hg ranged from 1.3 to 3.4 ng L(-1). During all sampling events when reducing conditions were only sufficient to promote NO₃(-) reduction (or denitrification) ( = 5, late fall 2009, winter 2010), MeHg concentrations decreased in the reactor and it was a net sink for MeHg (mean flux of -5.1 μg m(-2) yr(-1)). During all sampling events when SO₄(2-) reducing conditions were present ( = 6, early fall 2009, spring 2010), MeHg concentrations increased in the reactor and it was a strong source of MeHg to the stream (mean flux of 15.2 μg m(-2) yr(-1)). Total Hg was consistently removed in the reactor (10 of 11 sampling events) and was correlated to the total suspended sediment load ( r² = 0.69), which was removed in the reactor by physical filtration. This study shows that organic carbon bioreactors can be a strong source of MeHg production when SO₄(2-) reducing conditions develop; however, maintaining NO₃-N concentrations > 0.5 mg L suppresses the production of MeHg.

Sources of nitrate yields in the Mississippi River Basin

Riverine nitrate N in the Mississippi River leads to hypoxia in the Gulf of Mexico. Several recent modeling studies estimated major N inputs and suggested source areas that could be targeted for conservation programs. We conducted a similar analysis with more recent and extensive data that demonstrates the importance of hydrology in controlling the percentage of net N inputs (NNI) exported by rivers. The average fraction of annual riverine nitrate N export/NNI ranged from 0.05 for the lower Mississippi subbasin to 0.3 for the upper Mississippi River basin and as high as 1.4 (4.2 in a wet year) for the Embarras River watershed, a mostly tile-drained basin. Intensive corn (Zea mays L.) and soybean [Glycine max (L.) Merr.] watersheds on Mollisols had low NNI values and when combined with riverine N losses suggest a net depletion of soil organic N. We used county-level data to develop a nonlinear model ofN inputs and landscape factors that were related to winter-spring riverine nitrate yields for 153 watersheds within the basin. We found that river runoff times fertilizer N input was the major predictive term, explaining 76% of the variation in the model. Fertilizer inputs were highly correlated with fraction of land area in row crops. Tile drainage explained 17% of the spatial variation in winter-spring nitrate yield, whereas human consumption of N (i.e., sewage effluent) accounted for 7%. Net N inputs were not a good predictor of riverine nitrate N yields, nor were other N balances. We used this model to predict the expected nitrate N yield from each county in the Mississippi River basin; the greatest nitrate N yields corresponded to the highly productive, tile-drained cornbelt from southwest Minnesota across Iowa, Illinois, Indiana, and Ohio. This analysis can be used to guide decisions about where efforts to reduce nitrate N losses can be most effectively targeted to improve local water quality and reduce export to the Gulf of Mexico.

Denitrification in wood chip bioreactors at different water flows

Subsurface drainage in agricultural watersheds exports a large quantity of nitrate-nitrogen (NO(3)-N) and concentrations frequently exceed 10 mg L(-1). A laboratory column study was conducted to investigate the ability of a wood chip bioreactor to promote denitrification under mean water flow rates of 2.9, 6.6, 8.7 and 13.6 cm d(-1) which are representative of flows entering subsurface drainage tiles. Columns were packed with wood chips and inoculated with a small amount of oxidized till and incubated at 10 degrees C. Silicone sampling cells at the effluent ports were used for N(2)O sampling. (15)Nitrate was added to dosing water at 50 mg L(-1) and effluent was collected and analyzed for NO(3)-N, NH(4)-N, and dissolved organic carbon. Mean NO(3)-N concentrations in the effluent were 0.0, 18.5, 24.2, and 35.3 mg L(-1) for the flow rates 2.9, 6.6, 8.7, and 13.6 cm d(-1), respectively, which correspond to 100, 64, 52, and 30% efficiency of removal. The NO(3)-N removal rates per gram of wood increased with increasing flow rates. Denitrification was found to be the dominant NO(3)-N removal mechanism as immobilization of (15)NO(3)-N was negligible compared with the quantity of (15)NO(3)-N removed. Nitrous oxide production from the columns ranged from 0.003 to 0.028% of the N denitrified, indicating that complete denitrification generally occurred. Based on these observations, wood chip bioreactors may be successful at removing significant quantities of NO(3)-N, and reducing NO(3)-N concentration from water moving to subsurface drainage at flow rates observed in central Iowa subsoil.

In-stream bioreactor for agricultural nitrate treatment

Nitrate from agricultural activity contributes to nutrient loading in surface water bodies such as the Mississippi River. This study demonstrates a novel in-stream bioreactor that uses carbonaceous solids (woodchips) to promote denitrification of agricultural drainage. The reactor (40 m3) was trenched into the bottom of an existing agricultural drainage ditch in southern Ontario (Avon site), and flow was induced through the reactor by construction of a gravel riffle in the streambed. Over the first 1.5 yr of operation, mean influent NO3-N of 4.8 mg L(-1) was attenuated to 1.04 mg L(-1) at a mean reactor flow rate of 24 L min(-1). A series of flow-step tests, facilitated by an adjustable height outlet pipe, demonstrated that nitrate mass removal generally increased with increasing flow rate. When removal rates were not nitrate-limited, areal mass removal ranged from 11 mg N m(-2) h(-1) at 3 degrees C to 220 mg N m(-2) h(-1) at 14 degrees C (n = 27), exceeding rates reported for some surface-flow constructed wetlands in this climatic region by a factor of about 40. Over the course of the field trial, reactor flow rates decreased as a result of silt accumulation on top of the gravel infiltration gallery. Design modifications are currently being implemented to mitigate the effects of siltation. In-stream reactors have the potential to be scaled larger and could be more manageable than attempting to address nitrate loading from individual tile drains. They could also work well in combination with other nitrate control techniques.