Geostatistical analysis of tritium, groundwater age and other noble gas derived parameters in California

Nitrate trend reversal in Dutch dual-permeability chalk springs, evaluated by tritium-based groundwater travel time distributions

Historical use of fertilizer and manure on farmlands is known to have a lasting impact on ecosystems and water resources, but few studies assess the legacy of nitrate pollution on groundwater and surface water after farming applications were reduced. We studied the response of nitrate in spring water to a reduction of nitrogen fertilizer applications in agriculture realized since the mid-1980s. We assessed the travel time distribution of groundwater based on a time series of tritium measurements for 90 springs and small brooks that drain a dual porosity chalk aquifer. The travel time distributions were constrained using the tritium data in combination with time series of nitrate concentrations, applying a shape-free travel time distribution model. A clear trend reversal of nitrate concentrations was observed and simulated for springs with a large fraction of young water (< 30 years old) whereas the nitrate response in springs with relatively older water was attenuated and delayed. We conclude that obtaining a time series of tritium data helps to constrain age distributions of water that is discharged from dual permeability aquifers. The fraction of water aged <30 years was a meaningful parameter to distinguish between different types of springs. Nitrate trends in springs that drain large fractions of young water (> 0.6) show higher peak concentrations, shorter lag-time between leaching and outflow peaks and steeper declines after trend reversal, relative to trends in springs which are dominantly fed by older groundwater. The study thus shows that the nitrate legacy of groundwater systems is strongly determined by the range of their travel time distributions, and trend reversal in receiving springs and surface waters may appear within 10 to 15 years after measures to reduce nitrate losses from farming.

Mobility and persistence of pesticides and emerging contaminants in age-dated and redox-classified groundwater under a range of land use types

Understanding groundwater contamination patterns is hampered by the heterogeneous groundwater age and redox status over the depth range typically sampled for identifying pesticides and emerging contaminants threats. This study explores depth patterns of groundwater age and redox status across various land use types, unraveling spatial and temporal trends of pesticides and emerging contaminants using data from groundwater quality monitoring in the south of the Netherlands. The Netherlands is an ideal testing ground due to its high population density and widespread groundwater contamination from multiple sources. 146 multi-level observation wells were age-dated using 3H/3He, and contaminant concentrations were analyzed based on recharge year, land use type, and redox conditions, mitigating uncertainties from spatial and depth-dependent variations in both groundwater age and redox status. Redox-recharge year diagrams were developed to visually evaluate contaminant patterns in relation to these factors and to assess concentration patterns in relation to contamination history. Most detections of pesticides, metabolites, and emerging contaminants occurred in the youngest recharge periods (2000-2010 and 2010-2020) and in agricultural areas. However, certain contaminants, including BAM, desphenyl-chloridazon, short-chain PFCAs, PFOA, and EDTA, were consistently found in older water and Fe- or SO4-reduced conditions, indicating their mobility and persistence in the regional groundwater system. Comparing the presence of contaminants in specific redox classes and recharge periods with known application or leaching history provides insights into retardation (e.g., PFOS) and degradation (e.g., 2-hydroxy-atrazine, benzotriazole), explaining lower detection frequencies in earlier recharge periods. Identifying recharge years from age-dated groundwater helps relate contaminants to farmland application or river water recharge periods, revealing leaching history and contamination origins. The presented framework has the potential to enhance the interpretation of large groundwater datasets from dedicated, short-screened observation wells, such as those from the Danish GRUMO network, the Dutch monitoring networks, and parts of the US National Water Quality Program.

Noble Gas Analyses to Distinguish Between Surface and Subsurface Brine Releases at a Legacy Oil Site

Attributing the sources of legacy contamination, including brines, is important to determine remediation options and to allocate responsibility. To make sound remediation decisions, it is necessary to distinguish subsurface sources, such as leaking oil and gas ("O&G") wells or natural upward fluid migrations, from surface releases. While chemical signatures of surface and subsurface releases may be similar, they are expected to imprint specific dissolved noble gas signatures, caused by the accumulation of terrigenic noble gases in subsurface leaks or re-equilibration of noble gases following surface releases. We demonstrate that only a historic surface release influenced the dissolved noble gas signature of groundwater in monitoring wells contaminated with brine near an abandoned O&G well, rather than subsurface leakage from the well. Elevated brine concentrations were associated with lower terrigenic helium concentrations, indicating re-equilibration with atmospheric helium at the surface during the release. Geophysical surveying indicating elevated salinity in surficial soils upgradient of the wells further supported the interpretation of the noble gas data. Eliminating the possibility that subsurface leakage was the source of the plume was critical to selecting the proper remedial action at the site, which otherwise may have included an unnecessary and costly well re-abandonment. This study demonstrates the use of noble gas analysis to compare potential sources of brine contamination in groundwater and to exclude subsurface leakage as a potential source in an oilfield.

Estimation of groundwater residence time with deeply-derived carbon mixture considered in California

Groundwater has experienced long-term overdraft due to drought and human activities in California, resulting in issues of land subsidence and groundwater anthropogenic contamination. As a useful indicator of groundwater renewal rate and its residence time, groundwater age has been conventionally investigated based on ¹⁴C content and ³H concentration. However, the influence of deeply-derived (endogenic) CO₂ mixture is not fully and quantitatively considered, although endogenic carbon contributes a large part of the dissolved inorganic carbon in groundwater. Combined with ³H concentration, both conventional and modified ¹⁴C-dating methods with endogenic CO₂ mixture considered are employed for groundwater age determination in California. On average, the conventional groundwater ¹⁴C apparent age is overestimated by ~4.9 kyr or ~26.2 %, causing groundwater recharge rate underestimation and aquifer recovery time overestimation by ~46.0 % and ~26.2 %, respectively. High ³H concentration indicates modern water mixture in more than one fifth of groundwater samples, including those with high modified ¹⁴C apparent age (> 12 kyr, i.e., fossil groundwater) in the Central Valley and southern California, which are generally considered not to be recharged by modern water. Modern water mixture in old groundwater can potentially bring anthropogenic contamination to these groundwater resources, which should be paid attentions by the government and the managers. The results have important implications in evaluation of groundwater replenishment and its susceptibility to modern contamination in California, and in groundwater resources estimation globally.

Modern groundwater reaches deeper depths in heavily pumped aquifer systems

Deep groundwater is an important source of drinking water, and can be preferable to shallower groundwaters where they are polluted by surface-borne contaminants. Surface-borne contaminants are disproportionately common in ‘modern’ groundwaters that are made up of precipitation that fell since the ~1950s. Some local-scale studies have suggested that groundwater pumping can draw modern groundwater downward and potentially pollute deep aquifers, but the prevalence of such pumping-induced downwelling at continental scale is not known. Here we analyse thousands of US groundwater tritium measurements to show that modern groundwater tends to reach deeper depths in heavily pumped aquifer systems. These findings imply that groundwater pumping can draw mobile surface-borne pollutants to deeper depths than they would reach in the absence of pumping. We conclude that intensive groundwater pumping can draw recently recharged groundwater deeper into aquifer systems, potentially endangering deep groundwater quality.

Decades-old groundwater is vulnerable to pollution from land uses. This work shows that decades old groundwater flows to deep depths where groundwater pumping is more intense, implying that groundwater pumping can endanger deep groundwater quality.

Effect of Groundwater Age and Recharge Source on Nitrate Concentrations in Domestic Wells in the San Joaquin Valley

Nitrate is one of the most abundant contaminants in groundwater globally, in the United States, and in California (CA). We studied well construction information, water chemistry, stable isotopes, and noble gases to understand how groundwater travel time and recharge source and mechanism control nitrate concentrations in domestic wells in the San Joaquin Valley (SJV), CA, a large semiarid, irrigated agricultural region. Using nonparametric statistics, we find a decreasing trend in nitrates with groundwater travel time and well depth. Samples collected from wells that are closer to rivers and that show indications of river water recharge, either low recharge temperature or low δ¹⁸O signature, have lower concentrations of nitrates than samples with isotopic signatures indicating mixed source or local precipitation recharge. The curbing effect of river water recharge on nitrate concentrations in domestic wells is similar for direct river recharge and water applied as irrigation. This suggests that irrigation with river water also has a diluting effect that reduces the concentration of nitrate found in groundwater. This conclusion supports the idea that flood-managed aquifer recharge may be considered for remediation of groundwater nitrate when designing replenishment of aquifers.

Isotope and chemical assessment of natural water in the Jaffna Peninsula in northern Sri Lanka for groundwater development aspects

The demand for fresh groundwater in the Jaffna peninsula in northern Sri Lanka, has been increasing as the only water source for domestic and agricultural needs. Isotopic and hydro-chemical assessment on groundwater recharge and quality was done with the objective of assisting groundwater management plans. Water samples were collected in pre and post monsoon periods for isotope analyses (²H, ¹⁸O and ³H) and major ions. Isotope data provide evidence that the groundwater is predominantly recharged by inter-monsoon (convectional) and North-East monsoon rains. Different residence times of shallow groundwater were found. The study revealed that the groundwater quality is mainly deteriorated due to dissolution of geogenic elements. However, in some instances, seawater intrusion and evaporation have caused lowering of groundwater quality. More saline groundwater is found in deeper levels in the karstic limestone aquifer system. Fresh water is available in coastal sandy aquifers in the eastern part of the peninsula. The impact of rainfall variations and agricultural activities are contributory factors for producing variations in the quality of groundwater. The current study suggests artificial groundwater recharge for the development and utilization of groundwater resources through small scale tanks/reservoirs or ponds in the areas where direct rain recharge occurs.

High arsenic groundwater in the Guide basin, northwestern China: Distribution and genesis mechanisms

High arsenic (As) groundwater has been found in Pliocene confined aquifers at depths from 100 to 300 m of the Guide basin, but little is known on the main hydrogeochemical processes leading to its elevated concentrations. Ninety-seven water samples and fifty-three sediment samples were collected for chemical and/or isotopic analysis. Concentrations of As in groundwater of confined aquifer range from 9.9 to 377 μg/L (average 109 μg/L), which generally show a sharply increasing trend along with NH₄⁺, HCO₃^-, CO₃^2- and TOC along the inferred flow path, while NO₃^-, SO₄^2-/Cl^- and redox potential (Eh) have decreasing trends. Results of sequential extraction show that As bound to amorphous and crystalline Fe oxide minerals are the main As forms, accounting for around 50% of total As in sediments. Reductive dissolution of As-bearing Fe(III) oxide minerals under reducing conditions in confined aquifers lead to the mobilization of As in groundwater. In addition, alkaline environment and high concentrations of HCO₃^- and CO₃^2- may make contributions to As enrichment in groundwater. High As groundwater in confined aquifer continuously flows out on the ground surface through tens of artesian wells, which may potentially contaminate low As groundwater in unconfined aquifer. Thus, further investigation is needed to evaluate long-term variations of water chemistry of low As groundwater and assess vulnerability of unconfined aquifer to As contamination.

Assessing aquifer vulnerability from lumped parameter modeling of modern water proportions in groundwater mixtures: Application to California's South Coast Range

Groundwater in agriculture-dominated regions of California has historically experienced nitrate pollution due to the application of excess nitrogen fertilizers. This study examines the nitrate pollution vulnerability of groundwater in sedimentary aquifers of California's South Coast Range using stepwise logistic regression (LR) modeling. Our results indicate an overall excellent model fit, but an acceptable statistical significance, according to a Wald statistic (p-Wald) cutoff of 0.1, for only two explanatory variables: (1) the dissolved oxygen (DO) concentration, and (2) the modern (i.e., less than ~60year old) water proportion (MWP) in the groundwater mixture. The latter parameter was estimated via Lumped Parameter Modeling (LPM) of groundwater tritium, helium and radiocarbon data that have been corrected for isotopic dilution and exchange using a modified Fontes and Garnier (F&G) approach. The observation that other explanatory variables on land cover (i.e., percentage of agricultural land use, abundance of septic tanks and leaking underground fuel tanks, etc.) were statistically insignificant points out the limitations of low-resolution land cover data in groundwater vulnerability assessments. Our results highlight the utility of quantitative groundwater age and mixing data to evaluate pollution probability in the saturated zone. The herein presented approach can thus provide valuable results in comparable settings where the availability of fertilizer application, crop nitrogen uptake, and soil texture data is limited.

A hybrid machine learning model to predict and visualize nitrate concentration throughout the Central Valley aquifer, California, USA

Intense demand for water in the Central Valley of California and related increases in groundwater nitrate concentration threaten the sustainability of the groundwater resource. To assess contamination risk in the region, we developed a hybrid, non-linear, machine learning model within a statistical learning framework to predict nitrate contamination of groundwater to depths of approximately 500m below ground surface. A database of 145 predictor variables representing well characteristics, historical and current field and landscape-scale nitrogen mass balances, historical and current land use, oxidation/reduction conditions, groundwater flow, climate, soil characteristics, depth to groundwater, and groundwater age were assigned to over 6000 private supply and public supply wells measured previously for nitrate and located throughout the study area. The boosted regression tree (BRT) method was used to screen and rank variables to predict nitrate concentration at the depths of domestic and public well supplies. The novel approach included as predictor variables outputs from existing physically based models of the Central Valley. The top five most important predictor variables included two oxidation/reduction variables (probability of manganese concentration to exceed 50ppb and probability of dissolved oxygen concentration to be below 0.5ppm), field-scale adjusted unsaturated zone nitrogen input for the 1975 time period, average difference between precipitation and evapotranspiration during the years 1971-2000, and 1992 total landscape nitrogen input. Twenty-five variables were selected for the final model for log-transformed nitrate. In general, increasing probability of anoxic conditions and increasing precipitation relative to potential evapotranspiration had a corresponding decrease in nitrate concentration predictions. Conversely, increasing 1975 unsaturated zone nitrogen leaching flux and 1992 total landscape nitrogen input had an increasing relative impact on nitrate predictions. Three-dimensional visualization indicates that nitrate predictions depend on the probability of anoxic conditions and other factors, and that nitrate predictions generally decreased with increasing groundwater age.