The effects of small dam removal on the distribution of sedimentary contaminants

Accumulation and transport of nutrient and pollutant elements in riparian soils, sediments, and river waters across the Thames River Watershed, Connecticut, USA

Understanding drivers of nutrient and pollutant elements (NPEs) in soils, sediments, and river water is important for protecting water resources and aquatic ecosystems. The objectives of this study were to quantify accumulation and transport of NPEs (P, As, Cd, Cu, Ni, Pb, and Zn) in riparian soils, sediments, river water, and watershed-scale exports within seven post-industrial subwatersheds of the Thames River, Connecticut, USA. Suspended sediments and river water samples were collected from February 2019 to January 2020. Arsenic concentrations in soil (6 to 18 mg kg-1) and sediments (8 to 85 mg kg-1) generally exceeded state and federal EPA quality targets but not river water. Elevated Pb 'hot spots' occurred in some riparian soils (>2000 mg kg-1) and sediments (>200 mg kg-1), but the other NPEs concentrations were below toxic thresholds. Riparian soil concentrations and watershed land cover were generally weak predictors for NPE concentrations in bottom sediments, suspended sediments, and river water. DOC, Mn, and Fe concentrations were important predictors for area-normalized dissolved and sediment-bound export of NPEs across the seven watersheds. Dissolved export was greater than sediment export for Mn, P, As, Cd, Cu, and Ni but not for Fe, Pb, and Zn. Watersheds with higher farmland had higher P river water concentrations, but the larger, more urbanized watershed had the highest total and area-normalized P export. An estuarine sediment core that captures sediment from the whole watershed and spans pre-industrial conditions through present shows that export of most NPEs has decreased since its peak, but all remain above baseline throughout the Thames River watershed. Future constraints on surface soil-river exchange and erosion inputs are needed to investigate rates of NPE sourcing to the watersheds.

Archimedes Screw Turbines: A Sustainable Development Solution for Green and Renewable Energy Generation—A Review of Potential and Design Procedures

Archimedes Screws Turbines (ASTs) are a new form of small hydroelectric powerplant that can be applied even in low head sites. ASTs offer a clean and renewable source of energy and are safer for wildlife and especially fish than other hydro generation options. As with other energy solutions, ASTs are not a global solution for all situations. However, in terms of sustainable development, ASTs can offer many economic, social, and environmental advantages that make them an important option for providing sustainable hydropower development. Archimedes screws can operate in low water heads (less than about 5 m) and a range of flow rates with practical efficiencies of 60% to 80% and can generate up to 355 kW of power. ASTs increase the number of suitable sites where it is possible to develop sustainable hydropower, including in undeveloped, hard to access regions and small communities. At many low head sites, ASTs may be more cost-effective, with lower installation and operating costs than alternative hydropower systems. An AST may also reduce the disturbance of natural sedimentation and erosion processes and have smaller impacts on fish and other fauna. ASTs can often be retrofit to existing unpowered dams or weirs, providing new hydropower capacity for very little marginal environmental impact. This review outlines the characteristics of ASTs, then discusses and analyzes how they could benefit the sustainability of hydropower development.

Spatial and temporal patterns of metallic pollution in Québec City, Canada: Sources and hazard assessment from reservoir sediment records

Québec City (QC, Canada) is an important urban center developed along the Saint-Charles River, at the confluence with the Saint-Lawrence River. Here, environmental issues related to pollution have been recently raised for sediments trapped upstream a dam built in the early 1970s. The major concern is about downstream transport of sediments and contaminants toward the Saint-Lawrence Estuary, a protected marine area of high socioeconomic value. This article deals with metallic contaminants in reservoir sediments collected along a longitudinal transect in the Saint-Charles River. The spatial and temporal patterns of metallic pollution have been assessed by the calculation of enrichment factors, geoaccumulation indexes, and metallic pollution index on 68 samples from a set of sediment cores and surface sediment samples. Severe to extreme pollutions are recorded with respect to silver (Ag), chromium (Cr), copper (Cu), mercury (Hg), and lead (Pb). Spatial analyses show contaminated samples are trapped in the downstream section of the river, where several point (industries, mall, harbor) and diffuse (dense urban habitat, road network) sources of pollution were evidenced using historical documents and multivariate statistics such as PCA/FA. A 50-yr sedimentary record indicates these metals were mainly delivered to the river system by the accumulation of fine-grained, organic-rich sediments during the 1970s and the 1980s. Since then, the commissioning of wastewater treatment plants in the city and environmental regulations likely played a key role to reduce the metallic yield in the Saint-Charles River. More recently, the river flow management within the reservoir favored the accumulation of much less contaminated sediments, burying the contamination. Yet, a significant environmental hazard remains if this sandy layer is removed by erosion, allowing for the remobilization and transport of contaminated sediments downstream toward the Saint-Lawrence River.

Conserving rare species can have high opportunity costs for common species

Conservation practitioners face difficult choices in apportioning limited resources between rare species (to ensure their existence) and common species (to ensure their abundance and ecosystem contributions). We quantified the opportunity costs of conserving rare species of migratory fishes in the context of removing dams and retrofitting road culverts across 1,883 tributaries of the North American Great Lakes. Our optimization models show that maximizing total habitat gains across species can be very efficient in terms of benefits achieved per dollar spent, but disproportionately benefits common species. Conservation approaches that target rare species, or that ensure some benefits for every species (i.e., complementarity) enable strategic allocation of resources among species but reduce aggregate habitat gains. Thus, small habitat gains for the rarest species necessarily come at the expense of more than 20 times as much habitat for common ones. These opportunity costs are likely to occur in many ecosystems because range limits and conservation costs often vary widely among species. Given that common species worldwide are declining more rapidly than rare ones within major taxa, our findings provide incentive for triage among multiple worthy conservation targets.

Reductions in fish-community contamination following lowhead dam removal linked more to shifts in food-web structure than sediment pollution

Recent increases in dam removals have prompted research on ecological and geomorphic river responses, yet contaminant dynamics following dam removals are poorly understood. We investigated changes in sediment concentrations and fish-community body burdens of mercury (Hg), selenium (Se), polychlorinated biphenyls (PCB), and chlorinated pesticides before and after two lowhead dam removals in the Scioto and Olentangy Rivers (Columbus, Ohio). These changes were then related to documented shifts in fish food-web structure. Seven study reaches were surveyed from 2011 to 2015, including controls, upstream and downstream of the previous dams, and upstream restored vs. unrestored. For most contaminants, fish-community body burdens declined following dam removal and converged across study reaches by the last year of the study in both rivers. Aldrin and dieldrin body burdens in the Olentangy River declined more rapidly in the upstream-restored vs. the upstream-unrestored reach, but were indistinguishable by year three post dam removal. No upstream-downstream differences were observed in body burdens in the Olentangy River, but aldrin and dieldrin body burdens were 138 and 148% higher, respectively, in downstream reaches than in upstream reaches of the Scioto River following dam removal. The strongest relationships between trophic position and body burdens were observed with PCBs and Se in the Scioto River, and with dieldrin in the Olentangy River. Food-chain length - a key measure of trophic structure - was only weakly related to aldrin body burdens, and unrelated to other contaminants. Overall, we demonstrate that lowhead dam removal may effectively reduce ecosystem contamination, largely via shifts in fish food-web dynamics versus sediment contaminant concentrations. This study presents some of the first findings documenting ecosystem contamination following dam removal and will be useful in informing future dam removals.

Particle-bound metal transport after removal of a small dam in the Pawtuxet River, Rhode Island, USA

The Pawtuxet River in Rhode Island, USA, has a long history of industrial activity and pollutant discharges. Metal contamination of the river sediments is well documented and historically exceeded toxicity thresholds for a variety of organisms. The Pawtuxet River dam, a low-head dam at the mouth of the river, was removed in August 2011. The removal of the dam was part of an effort to restore the riverine ecosystem after centuries of anthropogenic impact. Sediment traps were deployed below the dam to assess changes in metal concentrations and fluxes (Ag, Cd, Cr, Cu, Ni, Pb, and Zn) from the river system into Pawtuxet Cove. Sediment traps were deployed for an average duration of 24 days each, and deployments continued for 15 months after the dam was removed. Metal concentrations in the trapped suspended particulate matter dropped after dam removal (e.g., 460 to 276 mg/kg for Zn) and remained below preremoval levels for most of the study. However, particle-bound metal fluxes increased immediately after dam removal (e.g., 1206 to 4248 g/day for Zn). Changes in flux rates during the study period indicated that river volumetric flow rates acted as the primary mechanism controlling the flux of metals into Pawtuxet Cove and ultimately upper Narragansett Bay. Even though suspended particulate matter metal concentrations initially dropped after removal of the dam, no discernable effect on the concentration or flux of the study metals exiting the river could be associated with removal of the Pawtuxet River dam. Integr Environ Assess Manag 2017;13:675-685. Published 2016. This article is a US Government work and is in the public domain in the USA.

Assessing organic contaminant fluxes from contaminated sediments following dam removal in an urbanized river

In this study, methods and approaches were developed and tested to assess changes in contaminant fluxes resulting from dam removal in a riverine system. Sediment traps and passive samplers were deployed to measure particulate and dissolved polycyclic aromatic hydrocarbons (PAHs) and polychlorinated biphenyls (PCBs) in the water column prior to and following removal of a small, low-head dam in the Pawtuxet River, an urbanized river located in Cranston, RI, USA. During the study, concentrations of particulate and dissolved PAHs ranged from 21.5 to 103 μg/g and from 68 to 164 ng/L, respectively. Overall, temporal trends of PAHs showed no increases in either dissolved or particulate phases following removal of the dam. Dissolved concentrations of PCBs were very low, remaining below 1.72 ng/L at all sites. Particulate PCB concentrations across sites and time showed slightly greater variability, ranging from 80 to 469 ng/g, but with no indication that dam removal influenced any increases. Particulate PAHs and PCBs were sampled continuously at the site located below the dam and did not show sustained increases in concentration resulting from dam removal. The employment of passive sampling technology and sediment traps was highly effective in monitoring the concentrations and flux of contaminants moving through the river system. Variations in river flow had no effect on the concentration of contaminants in the dissolved or particulate phases, but did influence the flux rate of contaminants exiting the river. Overall, dam removal did not cause measurable sediment disturbance or increase the concentration or fluxes of dissolved or particulate PAHs and PCBs. This is due in large part to low volumes of impounded sediment residing above the dam and highly armored sediments in the river channel, which limited erosion. Results from this study will be used to improve methods and approaches that assess the short- and long-term impacts ecological restoration activities such as dam removal have on the release and transport of sediment-bound contaminants.