A Bayesian approach to infer nitrogen loading rates from crop and land-use types surrounding private wells in the Central Valley, California

European Geosciences Union (EGU) | May 7th, 2018

This study is focused on nitrogen loading from a wide variety of crop and land-use types in the Central Valley, California, USA, an intensively farmed region with high�

This study is focused on nitrogen loading from a wide variety of crop and land-use types in the Central Valley, California, USA, an intensively farmed region with high agricultural crop diversity. Nitrogen loading rates for several crop types have been measured based on field-scale experiments, and recent research has calculated nitrogen loading rates for crops throughout the Central Valley based on a mass balance approach. However, research is lacking to infer nitrogen loading rates for the broad diversity of crop and land use types directly from groundwater nitrate measurements. Relating groundwater nitrate measurements to specific crops must account for the uncertainty about and multiplicity in contributing crops (and other land uses) to individual well measurements, and for the variability of nitrogen loading within farms and from farm to farm for the same crop type. In this study, we developed a Bayesian regression model that allowed us to estimate land-use-specific groundwater nitrogen loading rate probability distributions for 15 crop and land-use groups based on a database of recent nitrate measurements from 2149 private wells in the Central Valley. The water and natural, rice, and alfalfa and pasture groups had the lowest median estimated nitrogen loading rates, each with a median estimate below 5 kg N ha -1 yr -1. Confined animal feeding operations (dairies) and citrus and subtropical crops had the greatest median estimated nitrogen loading rates at approximately 269 and 65 kg N ha -1 yr-1, respectively. In general, our probability-based estimates compare favorably with previous direct measurements and with mass-balance-based estimates of nitrogen loading. Nitrogen mass-balance-based estimates are larger than our groundwater nitrate derived estimates for manured and non-manured forage, nuts, cotton, tree fruit, and rice crops. These discrepancies are thought to be due to groundwater age mixing, dilution from infiltrating river water, or denitrification between the time when nitrogen leaves the root zone (point of reference for mass-balance derived loading) and the time and location of groundwater measurement.

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

Science of the Total Environment (Elsevier) | June 9th, 2017

Intense demand for water in the Central Valley of California and related increases in groundwater nitrate concentration threaten the sustainability of the groundwater res

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 50 ppb and probability of dissolved oxygen concentration to be below 0.5 ppm), 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.

Addressing Nitrate in California’s Drinking Water Technical Report 1: Project and Technical Report Outline

University of California, Davis (UC Davis) | July 2nd, 2012

Addressing Nitrate in California’s Drinking Water Technical Report 2: Nitrogen Sources and Loading to Groundwater

University of California, Davis (UC Davis) | July 2nd, 2012

Nitrate loading to groundwater in the Tulare Lake Basin and Salinas Valley is widespread and chronic, and is overwhelmingly the result of crop and animal agricultural ac

Nitrate loading to groundwater in the Tulare Lake Basin and Salinas Valley is widespread and chronic, and is overwhelmingly the result of crop and animal agricultural activities. Urban wastewater, septic systems, and other sources may have significant localized impact. Due to long transit times, the impact on groundwater resources is a legacy for years and decades to come.

Addressing Nitrate in California’s Drinking Water Technical Report 3: Nitrogen Source Reduction to Protect Groundwater Quality

University of California, Davis (UC Davis) | July 2nd, 2012

Although reduction of anthropogenic loading of nitrate to groundwater aquifers will not reduce contamination in the short term (due to long travel times), reduction effo

Although reduction of anthropogenic loading of nitrate to groundwater aquifers will not reduce contamination in the short term (due to long travel times), reduction efforts are a critical component of any long term solution to the problem of high nitrate in drinking water. Technologies are available for reducing the transfer of nitrate to groundwater from surface sources. Such technologies involve (1) reducing the amount of nitrogen (N) discharged or applied to the land and (2) controlling the amount of water applied to land which serves as the carrier of nitrate. Many of these source control methods require changes in land management and upgrading of infrastructure.

Addressing Nitrate in California’s Drinking Water Technical Report 4: Groundwater Nitrate Occurrence

University of California, Davis (UC Davis) | July 2nd, 2012

This report reviews the hydrogeology and groundwater quality in the Tulare Lake Basin (TLB) and Salinas Valley (SV). We also assembled groundwater quality data from near

This report reviews the hydrogeology and groundwater quality in the Tulare Lake Basin (TLB) and Salinas Valley (SV). We also assembled groundwater quality data from nearly two dozen local, state, and federal agencies and other sources into a dataset, here referred to as the (Central) California Ambient Spatio-Temporal Information on Nitrate in Groundwater (CASTING) dataset. The dataset combines nitrate concentrations from 16,709 individual samples taken at 1,890 wells in the Salinas Valley (SV) and from 83,375 individual samples taken at 17,205 wells in the Tulare Lake Basin (TLB) collected from the 1940s to 2011, accounting for a total of 100,084 samples from 19,095 wells. Almost 70% of these samples were collected from 2000 to 2010; only 15% of the samples were collected prior to 1990. Half of all wells sampled had no recorded samples prior to 2000.

Addressing Nitrate in California’s Drinking Water Technical Report 5

University of California, Davis (UC Davis) | July 31st, 2012

This report explores methods and costs of remediation of groundwater nitrate contamination in the Tulare Lake Basin and Salinas Valley. Groundwater cleanup, or remediati

This report explores methods and costs of remediation of groundwater nitrate contamination in the Tulare Lake Basin and Salinas Valley. Groundwater cleanup, or remediation, is one of the most difficult actions in the environmental sciences, even when done on the scale of a small contaminant plume (1000s of cubic meters). Remediation of entire groundwater basins has never been attempted at the scale and depths of the Salinas Valley and Tulare Lake Basin, on the order of billions of cubic meters. This analysis shows that direct remediation to remove nitrate from large groundwater basins is extremely costly and not technically feasible. In situ remediation, though technologically infeasible as a regional remedy, is appropriate in certain areas of shallow groundwater with high contaminant levels. Traditional pump and treat (ex situ) methods are too slow to produce results on the regional scale in an acceptable time frame, prohibitively expensive, and impractical to implement. A novel form of pump-and-treat remediation is possible in the study area, but not yet widely practiced. By monitoring the nitrogen content of pumped irrigation water, and taking that ambient nutrient source into account when calculating additive fertilizer amounts, farmers can reduce the amount of nitrogen input to the aquifer. Such an approach is not unlike phytoremediation and is herein called “pump-and fertilize.” Pump-and-fertilize is part of an effective nutrient management program. Due to the nature of irrigation with groundwater, it is as much a source reduction method as it is a form of groundwater remediation. Hot spot and pump-and-fertilize remediation alone will not solve the problem of groundwater nitrate contamination. A new approach is needed that combines regional groundwater and nitrogen management strategies. This approach would include monitoring, source reduction, maximization of clean groundwater recharge across agricultural landscapes, pump-and-fertilize, and in situ treatment targeted at shallow, high-concentration plumes to place regional water quality on a trajectory toward improvement. Modeling and monitoring of regional groundwater quality are integral to an effective long-term management strategy.

Addressing Nitrate in California’s Drinking Water Technical Report 6: Drinking Water Treatment for Nitrate

University of California, Davis (UC Davis) | July 2nd, 2012

The purpose of the first three sections of this document is to provide a detailed guide to the current state of nitrate treatment alternatives that can be used as a refe

The purpose of the first three sections of this document is to provide a detailed guide to the current state of nitrate treatment alternatives that can be used as a reference tool for the drinking water community. The remainder of this document focuses on nitrate treatment of drinking water in California and specifically in the Tulare Lake Basin and the Salinas Valley.

Addressing Nitrate in California’s Drinking Water Technical Report 8: Regulatory and Funding Options for Nitrate Groundwater Contamination

University of California, Davis (UC Davis) | July 2nd, 2012

In the long-term, many regulatory options could reduce nitrate contamination from nonpoint sources, but some are preferable to others in terms of abatement costs; monito

In the long-term, many regulatory options could reduce nitrate contamination from nonpoint sources, but some are preferable to others in terms of abatement costs; monitoring and enforcement costs; information requirements; and the ability to raise revenue. Overall, regulation of fertilizer is easier to design, administer, and enforce than regulation of diffuse nitrate leachate discharges to groundwater. Market-based regulatory approaches (of fertilizer application or of nitrate leachate) often can achieve a water quality target at a lower cost than prescriptive standards. However, no regulatory option is without its drawbacks: e.g., regulation of fertilizer may not achieve the drinking water quality objective at all drinking water systems. Despite the existence of funding programs for safe drinking water, additional funding sources are needed. A fertilizer fee is a promising funding source that also creates incentives for dischargers to use fertilizer and water more efficiently. Farmers are currently exempt from a sales tax on fertilizer (if applied to land used for food or for feed for food animals) (CA State Board of Equalization 2004), so if this fee were set at a sales tax rate of 7.5% on the cost of nitrogen, it has the potential to raise an estimated $28 million annually, 12 nearly enough to provide alternative water supplies to the highly susceptible populations in the study area.

Addressing Nitrate in California’s Drinking Water with a Focus on Tulare Lake Basin and Salinas Valley Groundwater

University of California, Davis (UC Davis) | March 1st, 2012

In 2008, Senate Bill SBX2 1 (Perata) was signed into law (Water Code Section 83002.5), requiring the State Water Resources Control Board (State Water Board), in consultat

In 2008, Senate Bill SBX2 1 (Perata) was signed into law (Water Code Section 83002.5), requiring the State Water Resources Control Board (State Water Board), in consultation with other agencies, to prepare a Report to the Legislature to “improve understanding of the causes of [nitrate] groundwater contamination, identify potential remediation solutions and funding sources to recover costs expended by the State to clean up or treat groundwater, and ensure the provision of safe drinking water to all communities.” The University of California prepared this Report under contract with the State Water Board as it prepares its Report to the Legislature.  This executive summary focuses on major findings and promising actions. Details can be found in the Main Report and eight accompanying Technical Reports. Technical Report 1: Project and Technical Report Outline (Version July 2012) Technical Report 2: Nitrogen Sources and Loading to Groundwater (Version July 2012) Appendix, Technical Report 2: Appendix Figures to Technical Report 2 (Version July 2012) - 84 MB (large file) Technical Report 3: Nitrogen Source Reduction to Protect Groundwater Quality (Version July 2012) Technical Report 4: Groundwater Nitrate Occurrence (Version July 2012) Technical Report 5: Groundwater Remediation and Management for Nitrate (Version July 2012) Technical Report 6: Drinking Water Treatment for Nitrate (Version July 2012) Technical Report 7: Alternative Water Supply Options for Nitrate Contamination (Version July 2012) Technical Report 8: Regulatory and Funding Options for Nitrate Groundwater Contamination U.C. Davis Groundwater Nitrate Project links and updates