Presentations 2016

Groundwater nitrate concentrations in the Permo-Triassic aquifer of the Eden Valley, UK

British Geological Survey

https://www.youtube.com/watch?v=HVrKosUz2Sg&index=1&list=PLZC-5vj9_JAaQNkeKUtDhYOqJ3VZUX1BR

Groundwater nitrate concentrations in the Permo-Triassic aquifer of the Eden Valley, UK vary from less than 4 mg/l to in excess of 100 mg/l (as NO3). The variability is presumed to be due to land use and the main source of the nitrate is believed to be the nitrogen applied to grassland, both as slurry and as inorganic fertilizers. The main aim of this study was to estimate recharge rates and the timescale for water movement through the unsaturated zone and identify possible land use changes to reduce this influx of nitrate. Given the inherent uncertainties and limitations associated with the various methods for estimating recharge, it was proposed to use three different and independent methods and to compare the results obtained.The three methods proposed were:(i) to date the pore water profile within the unsaturated zone using the historical tracer tritium(ii) to date the pore water profile within the unsaturated zone using nitrate and chloride, released from the soil following the change in land-use from rough grazing to intensive pasture.(iii) To estimate recharge using a soil moisture water balance approach.The average recharge rate was found to be in the range 424-468 mm/y and the rate of water movement through the unsaturated zone is c. 3.5-3.85 m/y. Based on this estimate of water movement in the unsaturated zone, the travel time for recharge to migrate from the soil to the water table (or the delay imposed by the unsaturated zone) over the highest ground (where unsaturated zone thickness can be in excess of 175m) the travel times could exceed 50 years. However, over large areas of the Eden valley, the recharge currently arriving at the water table is of post 1990 origin. Thus an important conclusion is that, over most of the Eden valley, nitrate concentrations arriving at the water table are unlikely to substantially Increase but the effects of any BMPs are unlikely to be seen in the saturated zone for a significant number of years.

A combined approach for understanding nitrogen loading to groundwater from a field under potato production in Prince Edward Island

Environment and Climate Change Canada & Agriculture and Agri-Food Canada

https://www.youtube.com/watch?v=MYrsH383Yxk&list=PLZC-5vj9_JAaQNkeKUtDhYOqJ3VZUX1BR&index=2

Agricultural practices associated with potato production, the most important agricultural commodity in Prince Edward Island (PEI), have potential impacts on groundwater quality - the sole source of drinking water for the province. Groundwater with higher nitrogen loading can also be associated with the increased frequency of anoxic events in downgradient coastal ecosystems. In many cases, soil nitrogen cycling processes, which contribute significantly to the amount and form of nitrogen available for transport to groundwater, are neglected when nitrogen loading from agricultural fields is estimated. In this study, a preliminary model developed in Root Zone Water Quality Model (RZWQM, USDA-ARS) has been initiated for a three year potato rotation (potato-barley-red clover) implemented on a small field located at AAFC’s Harrington Research Farm, located 10 km north of Charlottetown. The nitrogen leached from the soil profile is further compared to nitrogen loading to groundwater derived based on water balance calculations, groundwater level dynamics and direct measurements of nitrogen concentrations in the aquifer. The soil at the study site is about one meter thick and is underlain by a thick (8 m) overburden derived from parental red sandstone bedrock, the dominant consolidated deposit across the province. Hydraulic conductivity in both the unsaturated and saturated zones is relatively high and previous investigations show that both the bedrock and the overburden are fractured, with fracture orientation being predominantly horizontal. The water table is about 16 m below ground and shows annual level amplitudes of 1-2 meters. In soil, the main inputs of nitrogen come from fertilizer application (150 kg N ha-1 y-1 for potatoes, 51 kg N ha-1 y-1 for barley with no fertilizer applied for clover) and mineralization of soil organic matter, which ranges between 42 and 52 kg N ha-1 y-1, depending on the rotation phase. In terms of nitrogen removal from soil, plant uptake is the most important process (126 kg N ha-1 y-1 under potatoes, 83 kg N ha-1 y-1 under barley and 23 kg N ha-1 y-1 under clover) and is followed by leaching (63.5 kg N ha-1 y-1 under potatoes, 50 kg N ha-1 y-1 under barley and 29.5 kg N ha-1 y-1 under clover), with all other losses being insignificant. The monthly dynamics of the nitrogen leaching is well reflected by the nitrogen loading calculated using a series of monitoring wells located downgradient of the study site. The loadings estimated using nitrate concentrations measured in these wells are 2-3 times lower than the amount of nitrogen leached through the bottom of the soil profile, which could be attributed to the mixing with the lower nitrogen concentrations water from the unconfined regional aquifer. The quick response of groundwater loads confirms that a portion of the nitrogen leaving the bottom of the root zone travel at a fast rate through the overburden.

Efficient data-driven estimation of nitrate transport and reactions in groundwater using a vertical flux model

US Geological Survey

https://www.youtube.com/watch?v=2Eb7z4oaDsk&index=3&list=PLZC-5vj9_JAaQNkeKUtDhYOqJ3VZUX1BR

Nitrate contamination in shallow groundwater extends across large regions of agricultural land around the world. Predictions of vertical nitrate migration are difficult because the effects of input histories can appear similar to the effects of reactions. Furthermore, nitrate reactive transport at large scales can be difficult to capture with detailed numerical models. Towards addressing these issues, we developed a simple modeling framework called the Vertical Flux Model (VFM) to analyze nitrate input histories and future concentrations in groundwater based on well information and measured tracer concentrations including dissolved oxygen, nitrate, atmospheric tracers of groundwater age, and dissolved gases. Application of the method yielded information about reaction rates and eventual depth of migration of nitrate at 14 detailed study sites across the US. Under the current N application rates and hydrogeochemical conditions, downward migration of the nitrate front will continue at seven sites with low denitrification rates (zero order rate constant less than 0.2 mg/L/yr). Comparison of results among the 14 sites suggests that rates of oxygen reduction and denitrification are correlated, and that denitrification rates tend to exceed oxygen reduction rates on an electron equivalent basis. Preliminary results from regional VFMs in California and Wisconsin confirm the correlation of rates, suggesting that that oxygen reduction rates, which are relatively easy to obtain, can serve as a proxy for denitrification rates in some regional studies. The applications of the VFM indicate that the method gives an efficient and flexible means of characterizing current and future nitrate fate and vertical transport in groundwater.

Decadal Changes in Agricultural Contaminants in Groundwater in the United States, 1988-2015

U.S. Geological Survey

https://www.youtube.com/watch?v=aazhtiQh1f0&list=PLZC-5vj9_JAZr3YYNllWhbFRapl6mAnpc&index=1

The U.S. Geological Survey, National Water-Quality Assessment (NAWQA) Project has been evaluating groundwater quality with respect to agricultural and other contaminants for more than 25 years. A key element of these studies has been resampling of networks of wells to evaluate changes in water quality on a decadal scale. Results from the period of 1988-2000 are compared to results from the period of 2000-2012. An interactive web-based mapping tool has been developed to illustrate these changes. This tool allows the user to display the statistical results for groundwater networks for 24 constituents, including agricultural contaminants such as nitrate, phosphorus, and several pesticide compounds. In addition to the comparison of data between the first and second decade of sampling, data from a third decade of sampling are available for those networks that have been sampled from 2013 to 2015. Highlights of the findings from the comparison of the first 2 decades include significant increases in concentrations of nitrate and the atrazine degradate deethylatrazine in about 21 percent and 25 percent of the 67 networks, respectively. Dissolved solids and chloride concentrations also increased in a large percentage of networks but have many sources in addition to agricultural activity. Other sampling and analysis has been done to enhance the understanding of the decadal-scale changes. Biennial sampling took place for a 10-year period on a subset of wells in each network, and can be used to help determine whether the decadal changes are part of a long-term trend, or to illustrate the timing of a trend reversal. Groundwater age-dating and flow modeling also provide perspective on the time frame in which changes might be expected, given a change in source inputs. The NAWQA project has also implemented continuous (daily) monitoring at a subset of wells across the country in order to understand changes that take place on a time scale that is less than a decade. In addition, wells are being sampled along a vertical profile in selected areas to help understand changes that take place on a time scale greater than a decade.

The California Nitrogen Assessment: Implications for the Future of Groundwater Resources

University of Colorado

https://www.youtube.com/watch?v=TCErhkhl6Zw&list=PLZC-5vj9_JAZr3YYNllWhbFRapl6mAnpc&index=2

Nitrogen (N) plays a vital role in sustaining the agricultural economy of California, as well as the global food supply. However, environmental losses of reactive N also have adverse impacts on other important ecosystem services by diminishing water and air quality and contributing to climate change. Despite the important benefits and tradeoffs of N in human and ecological systems, there remains a paucity of quantitative knowledge on the interrelationships between California’s N flows and the various ecosystem services that contribute to the public good. To fill this knowledge gap the Agricultural Sustainability Institute at UC Davis conducted a comprehensive integrated assessment of N in California to establish a baseline of credible information on the sources, sinks and flows of N into, out of, and within the state. This is the first comprehensive accounting of nitrogen flows, practices, and policies for California agriculture at the statewide level. We present an overview of results of a statewide N mass balance, focusing particular attention on losses of NO3 to groundwater. Leaching of NO3 to groundwater is a large flow of N within the state, accounting for roughly 16% of total N imports, with approximately 90% of the NO3 originating from crop and livestock production. Because N emissions from agricultural sources are geographically dispersed, cannot be easily observed, and are difficult to precisely control, this problem presents unique challenges for effective policy design. A suite of integrated practice and policy solutions may be needed to achieve both adequate source control and mitigation of the existing N contamination within reasonable time frames. We provide an overview of available policy instruments for nonpoint source pollution control and examine specific outcomes when these mechanisms have been implemented to control nitrogen pollution in practice. Policy characteristics are then organized into a coherent methodology for assessing candidate policies for controlling nitrogen emissions from agricultural sources in California.

Spatial and temporal variability of nitrate in Wisconsin’s groundwater

University of Wisconsin - Extension & University of Wisconsin - Stevens Point

https://www.youtube.com/watch?v=veBSJ2Q2bG8&list=PLZC-5vj9_JAZr3YYNllWhbFRapl6mAnpc&index=3&spfreload=10

Nitrate is a widespread groundwater contaminant found in agricultural regions throughout the world. Wisconsin, USA is located in a humid-continental climate. Nearly one-quarter of its land base is classified as farmland; primary cropping systems include corn (32% of farmland), hay and forage (27%) and soybean (13%). Wisconsin is fortunate that groundwater is generally abundant, freshwater with low salinity and easily accessible. Major aquifers include materials from the Quaternary (sand/gravel), Ordovician (dolomite/sandstone) and Cambrian (sandstone) periods. With over 850,000 domestic wells and 97% of communities relying on public water supply wells, groundwater is the primary water supply for nearly 70% of the state’s 5.8 million residents and many of its industries. Because of the heavy reliance on groundwater wells – both domestic and public – Wisconsin has amassed an extensive set of well water data. The extensive spatial distribution of data has helped create a detailed picture of groundwater quality across the state. The information is useful for helping communities to understand the relationship between land-use in groundwater. Here we investigate what is known about the extent of nitrate pollution in Wisconsin’s groundwater as well as trends in nitrate concentrations over time. Post well construction, private wells are not required to test or report results; however some of that data has been captured by UW-Extension, state agencies and county health departments. Data from these sources has been aggregated into the Wisconsin Well Water Quality Viewer. It contains information on nitrate for 133,882 water samples spanning 1985 to present. Statewide 10% of well samples exceed the 10 milligrams per liter nitrate-nitrogen drinking water standard; while 42% contain concentrations above 2 milligrams per liter providing evidence that land-use is impacting well water quality. Wells located in agricultural regions and installed into sand/gravel aquifers or shallow carbonate rock aquifers are particularly impacted with nitrate. Because most private wells aren’t sampled regularly the data isn’t particularly useful at a local scale for understanding how water quality has changed over time. As a way to investigate temporal variability, we turned to data from transient non-community water systems (bars, restaurants, churches, etc.) which are required to regularly test water for nitrate and report results. This dataset provided a unique opportunity to evaluate temporal changes in shallow groundwater nitrate concentrations using linear regression. While the temporal analysis indicates nitrate statewide has increased in more areas than decreased, the shallow groundwater accessed by 7,447 (87%) of TNC wells did not indicate a significant trend either way. Of the 13% wells with a trend, 726 increased (8%) while 421 decreased (5%). This information will help Wisconsin prioritize outreach strategies to private well users and assist communities in those areas most affected by elevated and potentially increasing nitrate concentrations in groundwater. It may also help other regions with less detailed nitrate data communicate the role of land-use, soils and geology on groundwater quality.

Bayesian Nitrate Source Apportionment to Individual Groundwater Wells in the Central Valley by Use of Elemental and Isotopic Tracers

UC Davis

https://www.youtube.com/watch?v=0TOt_PpW9kQ&index=4&list=PLZC-5vj9_JAZr3YYNllWhbFRapl6mAnpc

Groundwater quality is a concern in alluvial aquifers that underlie agricultural areas, such as in the San Joaquin Valley of California. Nitrate from fertilizers and animal waste can leach to groundwater and contaminate drinking water resources. Dairy manure and synthetic fertilizers are prevailing sources of nitrate in groundwater for the San Joaquin Valley with septic waste contributing as a major source in some areas. The rural population in the San Joaquin Valley relies almost exclusively on shallow domestic wells (less than 150 m deep), of which many have been affected by nitrate. Knowledge of the proportion of each of the three main nitrate sources (manure, synthetic fertilizer, and septic waste) contributing to individual well nitrate can aid future regulatory decisions. Mixing models quantify the proportional contributions of sources to a mixture by using the concentration of conservative tracers within each source as a source signature. Deterministic mixing models are common, but do not allow for variability in the tracer source concentration or overlap of tracer concentrations between sources. In contrast, Bayesian mixing models treat source contributions probabilistically, building statistical variation into the inferences for each well. The authors developed a Bayesian mixing model on a pilot network of 56 private domestic wells in the San Joaquin Valley for which nitrogen, oxygen, and boron isotopes as well as nitrate and iodine were measured. Nitrogen, oxygen, and boron isotopes as well as iodine can be used as tracers to differentiate between manure, fertilizers, septic waste, and natural sources of nitrate (which can contribute nitrate in concentrations up to 4 mg/L). In this work, the isotopic and elemental tracers were used to estimate the proportional contribution of manure, fertilizers, septic waste, and natural sources to overall groundwater nitrate concentration in individual wells. Prior distributions for the four tracers for each of the four sources were estimated based on end member measurements, literature, or as a part of our previous work. The Bayesian method produces estimates of the fractional source contributions to each well, which can be compared to surrounding landuse types. Estimated source contributions were broadly consistent with nearby landuse types in this sample.

Groundwater pathways for nutrient transport from agricultural land to the Great Barrier Reef

Queensland University of Technology

https://www.youtube.com/watch?v=bV3hR8CyyNs&list=PLZC-5vj9_JAZr3YYNllWhbFRapl6mAnpc&index=5

The World Heritage listed Great Barrier Reef (GBR) off the northeast coast of Australia is the largest reef in the world. Unfortunately, agricultural production in GBR catchments over the past 150 years has contributed to a decline in water quality entering the GBR lagoon. Riverine discharge has been identified as the single largest source of nutrients to inshore areas of the GBR lagoon. However, the contribution of groundwater discharge to nutrient concentrations in rivers and streams in GBR catchments is currently uncertain.One of the GBR catchments of particular interest is the Lower Burdekin catchment. In this catchment, the predominant crop grown is sugarcane and there are ongoing concerns related to soil and water management. A significant body of research has been undertaken in this catchment, focussing mostly on paddock scale monitoring and modelling. In addition to this, a regional scale modelling toolkit has been developed which aims to support decision making for water management by addressing local groundwater management issues including: declining groundwater quality, rising water tables and increased discharge of poor quality groundwater to the environment.In recent years, there has been a significant increase in the monitoring of groundwater nutrient concentrations underlying agricultural land in the Lower Burdekin, including a program of voluntary monitoring by cane-growers where 962 samples were taken over a 12 month period. Around 40% of the groundwater samples from this voluntary monitoring program contained nitrate at concentrations > 5mg nitrate-N/L. The fate of this nitrogen remains unclear. Some preliminary research has been conducted in the Lower Burdekin into nitrogen transport both from aquifers directly to the marine environment and from aquifers to surface water. This research has identified the need to better define i) groundwater flow paths to the marine environment and the possible role of preferential flow paths; and ii) spatial and temporal variations in redox conditions in these environments and the processes that underlie them. Further research, incorporating isotopic analyses and geochemical modelling, is required to better understand the processes occurring and to improve estimates of nutrient fluxes to the GBR via groundwater.

Hydrogeochemical characterization in relation to nitrate concentrations in Central Valley (California, USA) domestic wells

Lawrence Livermore National Laboratory

https://www.youtube.com/watch?v=2-9qUJB_wXE&index=4&list=PLZC-5vj9_JAaQNkeKUtDhYOqJ3VZUX1BR

Agriculture and groundwater are both of vital socio-economic importance to the California Central Valley (USA). High nitrate concentrations from agricultural practices threaten the quality of domestic well drinking water. Hydrogeochemical characteristics of the aquifer can reduce or enhance the risk that nitrate or naturally occurring contaminants like arsenic or uranium exceed the Maximum Contaminant Level (MCL) at the domestic wells.We analyzed a data set of nitrate concentrations and isotopes, 3H/3He groundwater ages, major ion chemistry (including arsenic and uranium) and hydrological setting, collected from 200 domestic wells in the San Joaquin Valley. Nitrate exceeds the MCL in 44% of the wells, uranium in 17% of the wells and arsenic in 11% of the wells. 13% of the wells produce pre-modern groundwater (recharged prior to 1950) with less than 1 pCi/L tritium and 6% of the wells produce anoxic water (Fe > 0.05 mg/L). Nitrate concentrations correlate strongest with area of manure lagoons (R=0.45), corrals (0.41) or dairy manure application areas (0.26) within a 1.5 mile radius (Groundwater Nitrogen Loading Model input) but nitrate concentrations are difficult to predict reliably based on land use data alone. Nitrate negatively correlates with younger groundwater ages (R=-0.42) and with water table depth (R=-0.31),These findings are supported by kmeans cluster analysis performed on nitrate, mean groundwater age, and average depth to groundwater near each well. Wells with the greatest mean age (52.3 years) and relatively deep mean depth to groundwater (37.8 m) have the lowest mean nitrate concentrations (2.3 mg/L). Wells with relatively young mean age (9.6 years) and relatively shallow mean depth to groundwater (16.7 m) have the highest mean nitrate measurements (40.8 mg/L). Kmeans cluster analysis performed with groundwater age, nitrate, arsenic, and uranium reveals four groundwater contaminant groupings. One group represents groundwater with a mean age 51.1 years with low concentrations of all three contaminants. A second group represents old groundwater (mean age 46.6 years) with the lowest mean nitrate concentrations (1.65 mg/L) but with the greatest amount of arsenic (mean of 44.7 µg/L). Two groups contained mean nitrate concentrations above the MCL. The group with the greatest mean nitrate concentration (29.1 mg/L) also has the greatest mean uranium concentration (95.2 µg/L). In conclusion, predictions of nitrate, uranium and arsenic concentrations improve when hydrogeochemical parameters are included and are suited to predict the risk of wells to future contamination. The hydrogeochemical characterization also revealed risk factors for nitrate co-contaminants like uranium and natural contaminants like arsenic.