Groundwater Nitrate / SBX2 1 Project
Groundwater Nitrate / SBX2 1 Project
Groundwater Nitrate / SBX2 1 Project
University of California
Groundwater Nitrate / SBX2 1 Project

Questions and Answers


Why is nitrate a problem?

Nitrate in drinking water, at high levels, causes "blue baby syndrome" or methemoglobinema. This is an acute illness that can occur in unborn infants and young children. It leads to low blood oxygen levels and possibly suffocation. In adults, long-term exposure to high nitrate levels in drinking water is potentially also associated with thyroid dysfunction and cancer. Where groundwater is the source of drinking water, nitrate may exceed the national drinking water standard of nitrate (45 mg nitrate per liter or, equivalently, 10 mg of nitrate-measured-as-nitrogen per liter).

For surface water sources of drinking water, nitrate generally does not exceed the drinking water standard.  But nitrate in surface water - at levels much lower than the drinking water standard - may cause eutrophication. Eutrophication means that there are too many nutrients in the water, causing algal blooms, which in turn overuse the oxygen in streams, lakes, or oceans, ultimately leading to complete oxygen removal from these aquatic ecosystems ("hypoxia"), killing fish and other aquatic life (including the algae).

What are the sources of nitrate?

Nitrate is part of the earth's reactive nitrogen cycle. Nitrogen is a key ingredient in synthetic fertilizer. It is also part of any living materials (for example, proteins contain a lot of nitrogen). Human and animal waste, and dead plant and food  waste materials therefore also contain nitrogen, typically as organic nitrogen or as ammonium. Fertilizer, human/animal waste, or other plant/food waste (e.g., compost) is commonly applied to land (for example, in septic leach field, wastewater percolation ponds, biosolids applied to cropland, wastewater effluent applied to cropland, synthetic fertilizers applied to crop). Once in the soil, nitrogen will travel to groundwater as nitrate unless it is used by plants, released into the atmosphere, or washed out in overland runoff. It may also be stored temporarily in the soil root zone or in growing perennial plants (trees, vines).

Where are the nitrate sources?

Nitrate sources are effectively everywhere - agricultural lands, natural lands, urban areas with leaky sewer lines, septic leach fields, and wastewater percolation basins. Not all sources pollute groundwater. There is great variability in nitrate loading to groundwater between different sources, between different management practices, and from locale to locale due to variable natural conditions that affect the fate of nitrogen.

How much nitrate does an average cow produce?

An average milking cow produces about 1 lb per day of nitrogen in its urine and feces. Cow manure is stored (solids in piles, liquids in storage lagoons) and then land applied. Between 20% and 40% of the excreted nitrogen will end up in the atmosphere, the remainder is land applied on forage crops or other crops. A dairy with 1,000 milking cows has about 300,000 to 400,000 lbs of fertilizer nitrogen available for land application via manure.  Manure nitrogen is partly organic nitrogen, which is relatively difficult to manage as fertilizer. Part of manure nitrogen is ammonium, a commonly applied fertilizer.

How much nitrogen does a farmer use?

Nitrogen fertilizer applications depend, first, on whether the crop can "make" its own nitrogen (alfalfa, beans, other legume crops). If the crop needs an outside source of nitrogen, fertilizer is applied and the amount will depend mostly on the nitrogen demand of the crop. The more nitrogen is harvested with the crop, the larger the need for fertilizer. Grapes remove relatively little nitrogen with their harvest (less than 20 lb of nitrogen per acre per year). Where two non-leguminous forage crops are grown each year, or in vegetable crops, but also in nut crops (high protein content!) and some tree crops, relatively large amounts of nitrogen are harvested (200 to over 400 lb of nitrogen per acre per year).  Fertilizer applications must be somewhat higher than the amount of nitrogen removed in the harvest. The over-application depends on many factors including what crop is grown, the soil type, the irrigation type, management practices, and a grower's tradition.  Under good management practices, fertilizer applications are 120% to 150% of the amount harvested, although it is common that fertilizer nitrogen applications (including compost and other organic waste amendments) are 200% or more of the nitrogen harvested.

How much nitrate-nitrogen is 10 mg/L (the drinking water limit), when measured in lb of nitrogen per acre-foot of water?  The drinking water limit (10 mg of nitrate-measured-as-nitrogen per liter) is equivalent to 27 lb of nitrate-nitrogen in one acre-foot of water.  Groundwater recharge in irrigated agriculture is typically on the order of one acre-foot per acre. 

Why don't we just use well-head treatment for nitrate in well water used as drinking water?  This can be done and many public water systems have installed well-head treatment systems (either an ion exchange system or a reverse osmosis system). These are not inexpensive to buy and install, and there is a significant ongoing maintenance cost (filter maintenance and replacement).

What can I/we do about this problem?  Depending on where you are, what your water source is, and how you want or need to engage, you can:

  • test your own well (domestic well owner) to make sure that your water is safe;
  • make sure that your septic leach field (domestic homeowners) is well maintained and not immediately updgradient from your domestic well (source control);
  • engage in and with your community if it is facing a drinking water safety issue, get informed about the problem, about potential solutions to the problem, their pros and cons; engage with a community group or with your business/work community, if it can be part of the solution.

The following is a list of questions and answers from the local workshops held in the Tulare Lake Basin on May 3, 2012 and in Salinas on May 17, 2012, and from the SWRCB Hearing in Sacramento on May 23, 2012.


What was your peer review process? 

Preliminary results were presented to the Interagency Task Force in a public meeting in May 2011. Drafts of the report were first reviewed internally by the UC Davis Team, revised drafts of the technical reports were reviewed by nearly 30 technical and scientific experts - about 2-5 reviewers for each technical report. The reports were also reviewed by ITF members and the State Water Board.

Are the incidences of health impacts from nitrate in Tulare County reflective of overall groundwater quality for this constituency?

This study did not investigate health impacts in the study area. Impacts and suggested solutions were focused on meeting the current drinking water standard.

Why did you not separate the data for the Tulare Lake Basin from those for the Salinas Valley? 

The main report summarizes much of the detailed technical data and provides only some of the data separately for the Tulare Lake Basin and the Salinas Valley. Details for each of the two basins separately are described in the Technical Reports. The Technical Reports provide most analyses separately for the two basins. Some of the data are further broken down by groundwater sub-basin or county.


Is the resultant brine from reverse osmosis considered hazardous waste, and if so, who will be responsible for discarding it, and how will it be handled?

The appropriate disposal of waste brine or concentrate from nitrate treatment will depend on several factors including co-contaminants, the strength or concentration of the waste stream, and the volume of the waste stream. The presence of contaminants other than nitrate (e.g., arsenic, selenium, uranium, chromium and vanadium) in the waste stream can have a significant impact on brine or concentrate disposal options and costs; under such circumstances disposal to a hazardous waste facility may be required at a greater cost. Methods for disposal of waste brine or concentrate reported in the survey of nitrate treatment systems in CA include discharge to a septic tank and leach fields, to a wastewater treatment plant via a sewer connection, to irrigation ponds, to a brine line, and to a wastewater treatment plant via trucking. 

Based on the water quality characteristics of the waste stream, waste disposal options for a nitrate treatment system in a public water system will be considered in the planning and design phases to select the most appropriate fate of the waste stream under state and local regulations. On a residential scale, the waste stream from a Point-of-Entry (POE) or Point-of-Use (POU) system would typically be discharged through a sewer connection or, depending on brine volume and water quality characteristics, to a septic system. If a company is contracted to manage POU or POE treatment units, waste management should be included. The emergency regulations regarding the use of POU or POE treatment units for small public water systems in California indicate the need to address waste disposal as part of the development of an Operations and Maintenance Program for review by the California Department of Public Health.


Did you conduct an economic analysis of the cumulative impact of adding regulations on top of existing regulations?

The report did not specifically analyze the cost of regulations or the cost of multiple regulations. The report outlines current regulatory programs and potential options. It clearly identifies and compares, in general terms, the cost of such regulations for various options.  The report also estimates the cost of reducing nitrate loading from various sources, including agriculture, that arise from investments into additional infrastructure, education, labor, and monitoring. The report does discuss as a dilemma for the state that current policy has separate state agencies for nitrate, pesticide, and air pollution emissions from the same land use. Can this be streamlined to make it more effective?

What is the average annual revenue from agricultural production, in relation to the $30 billion cost of remediation indicated in the study?

The cost of remediation is excessively high and its technical feasibility at this scale questionable. Large scale, basin-wide remediation via pump-and-treat is not a viable option.

In regard to problem identification, do cost estimates consider application stationing and does groundwater recognize borders?

The study does not develop place-specific solutions. Instead, the study provides information on the use of specific management practices (agricultural nitrate loading) or specific drinking water supply options and outlines under what circumstances, which options are more promising.  These options take into consideration that groundwater is controlled by natural boundaries and the makeup of the aquifer systems in the Salinas Valley and Tulare Lake Basin.

If the mill tax on synthetic nitrogen is increased according to the study’s recommendation, is that likely to encourage growers to use dairy waste (effluent) in place of synthetic fertilizer, and will that have an increased impact on groundwater?

We did not evaluate the effect of taxing synthetic fertilizer only on potentially more widespread distribution of manure. In general, the effect on groundwater from additional use of manure in lieu of synthetic fertilizer depends on the farmer's management practices. It may or may not lead to changes in groundwater quality impacts. Only if the use of manure results in higher applications of nitrogen, when compared to applying only synthetic nitrogen fertilizer, groundwater nitrate leaching will increase (unless the harvested nitrogen increases proportionally). For a farmer, switching from synthetic fertilizer to manure as fertilizer is a significant shift in nutrient management practices.

The UC Davis study states that regulations alone are not adequate to resolve the nitrate problem. However, the Dairy General Order is quite new, and the Irrigated Lands Program is currently under development. Will these programs be allowed adequate time to prove their effectiveness or ineffectiveness?

These programs are highlighted in the report and are among several programs already in place. The analysis and options outlined in our report are not considered to be new alternatives to these existing programs, but may further inform these ongoing programs as they develop.

Will the suggested fees be broken down proportionately as to who will pay them, (i.e., agricultural industry only or all sources thought to cause nitrate contamination)?

We outline several options that the state or even a county may pursue. Some focus on polluters, some are general fees.

Should the State be doing more or less on the issue of nitrate in groundwater than they are currently doing?

The report outlines a number of promising actions for the State to more effectively and efficiently address nitrate in drinking water. Without any additional State efforts, the problem cannot be addressed.


How did you come up with your nitrogen loading numbers?

Nitrate loading was evaluated by various methods described in Technical Report 2:

  • Wastewater treatment plant and food processor data were collected from permits, monitoring reports, and, in some cases, through contact with facilities, to identify the amount of nitrogen in their wastewater, the fraction discharged to percolation basins, the fraction land applied, and the associated land area.  Loading from smaller wastewater treatment plants and from facilities for which no data were available was estimated based on collected data as described in Technical Report 2.
  • Septic system leaching was estimated based on average per person nitrogen excretion rates documented in the engineering literature and an analysis of septic system density in the study area.
  • Urban and golf course nitrate loading was estimated based on literature review and mass balance evaluations.
  • Agricultural nitrogen loading was based on a crop- and county-specific mass balance analysis.  The mass balance considered various field inputs of nitrogen (fertilizer, manure, effluent, biosolids, irrigation water nitrate, atmospheric deposition) and the nitrogen removal to harvest, losses to atmosphere and to runoff. The loading rates are considered long-term average loading rates and do not account for year-to-year, field-to-field, or farm-to-farm differences due to climate variability, soil variability, or farmer behavior. There are not sufficient data available for such an analysis.

Did you acknowledge the beneficial reuse and existing regulatory environment of land application of biosolids? 

This issue is discussed in Technical Report 2 (Chapter 6). The total N from biosolids was calculated based on literature estimates for % N, as 3.3% (dry wt.). All sources of nitrogen were tallied in the overall mass balance approach, and biosolids were included in this total. The Technical Report also details the relative amount of chemical fertilizer applied in addition to biosolids, WWTP/FP effluent, etc.

Farmers always cut back on synthetic nitrogen on manured ground. Why do you assume that farmers applying organic N (manure) continue to add synthetic fertilizer?

In surveys conducted prior to the 2007 Dairy General Order, dairy farmers in the Central Valley reported using significant amounts of synthetic fertilizer on their crops. Estimating the release of available N from organic sources is complicated by many factors making it difficult to replace synthetic fertilizer N with manure N at a 1:1 ratio. This difficulty in determining the nutrient value of manure contributes to total N applications often being higher than under a strictly synthetic fertilizer regime, while also providing significant benefits to soil quality.  Growers who use manure (or other organic amendments) primarily as a soil amendment may reduce fertilizer applications, but not at a 1:1 ratio when considering the total N content of manure (or other organic amendments). Clearly, further data is needed to make a more specific assessment of current practices.

You provided the 34% NUE, that is, the overall average of manure and synthetic fertilizer (plus other sources of N) combined. Have you conducted a separate analysis for manure-applied vs. non-manured acreage?

Yes, Appendix Table 7 in Technical Report 2 lists, for each crop, the estimated nitrogen typically applied and the estimated amount of nitrogen typically harvested. Chapter 3 of Technical Report 2 explains how these numbers were obtained. In general, these numbers reflect typically applied synthetic fertilizer. Chapter 4 of Technical Report 2 explains application of manure to forage crops and our assumptions about the amount of manure N that replaces synthetic fertilizer N on dairies.

How many farms, dairies, etc. were visited or consulted in developing the suggested groundwater contamination mitigation measures?

Five expert panels were convened to assemble, discuss, and evaluate a list of recommended practices that would support significant reductions of nitrate leaching to groundwater. Each panel included 2 Cooperative Extension advisors, 2-3 growers, and 2-3 crop management professionals or other industry personnel, such as certified crop advisors.  Mitigative potential, extent of use by local growers, and barriers to use of each practice were discussed in detail, as described in Technical Report 3 (Chapter 2).

How fast or slow does nitrate move through the soil profile, from time of application to the time it reaches the groundwater?

The movement of nitrate through the soil profile depends on the time of application, the subsequent timing and amount of rainfall and irrigation, and the rate of plant water and nitrate uptake. If nitrogen is applied in organic form or as ammonia, the rate of nitrate movement will also depend on how quickly organic nitrogen is mineralized to ammonia, and how quickly the ammonia is then converted to nitrate. Organic nitrogen and ammonia nitrogen are "sticky" and most of it remains in the root zone near the soil surface. But nitrate is very mobile and readily displacable by water movement.
Once nitrate leaves the root zone, the travel time to groundwater depends on the depth to groundwater and the amount of water percolating out of the root zone from irrigation and rainfall. Technical Report 4 (Chapter 6) provides a detailed analysis of the potential travel time of nitrate between the root zone and groundwater.

Was support for organic agriculture considered as a source reduction option? If so, please explain.

We did not specifically address organic agriculture in this report. Instead, Technical Report 3 considers specific recommended practices, some of which may already be practiced by farmers, that would at least partially address nitrate leaching.

If today’s growers are more conservative with fertilizer application, then why are state sales reported by CDFA still as high or higher than 10 or 20 years ago?

Many growers are harvesting higher yields today than they did 10 or 20 years ago, while keeping their fertilizer use approximately the same or even reducing fertilizer use in some cases.


Why are there high levels of groundwater nitrate along the foothill regions?

The Central Valley aquifer system tends to be more sandy along the eastern margin near the foothills.  There, it is also not very thick; hence, many wells are drilled to relatively shallow depths. The region is therefore very vulnerable to nitrate contamination of drinking water wells.

Is it possible that the estimated increase of nitrate in groundwater may have been skewed by an increase in drilling wells in contaminated areas?

The long-term trends seen in the groundwater nitrate data partially reflect not the drilling of new wells, but the fact that more wells are being tested in recent years, at least in some areas. In other areas, testing programs in the 1960s, 1970s, and 1980s on certain wells have since stopped. The trend analysis accounted for some of these changes, by breaking out data from separate programs, and by considering trends at individual wells in addition to trends in averages across multiple wells (see Technical Report 4)

Why do you expect a higher rate of future degradation of groundwater?

Groundwater nitrate loading has not diminished and in many areas has increased. The effect of that loading will continue to be felt in groundwater wells for years and decades.

How were background nitrate levels established, especially for northeast  Kern county? And, how do you define natural background nitrate?

We did not establish specific background nitrate levels. The U.S. Geological Survey typically uses nitrate levels of 9 mg/L, 13.5 mg/L or 18 mg/L as a threshold to differentiate between what is possibly natural nitrate and what is likely "anthropogenically influenced" nitrate.  We developed data for all these thresholds, but have focused on the 9 mg/L threshold, the 22.5 mg/L threshold (half of the MCL) and the 45 mg/L threshold (the MCL).

Did you conduct any testing of aged groundwater, or was aged groundwater used in any of the comparisons?

The USGS has done some analysis of the groundwater age distribution with depth and how it relates to nitrate levels. Further work is currently under way by several research groups (UC Davis, US Geological Survey, Lawrence Livermore National Laboratory).

Are there other areas of California that have high nitrogen levels, and did you project a cost estimate for the entire state?

There are other areas in California with high nitrate levels in groundwater. These include areas in coastal basins south of the Salinas Valley, the San Fernando Valley, the Chino basin, the Los Angeles greater area, the northern San Joaquin Valley, and some areas in the Sacramento Valley. We did not project cost estimates to these regions.

Have there been any isotope studies conducted to track nitrate sources, and if so, where?

Isotope studies have been conducted in the Salinas Valley and in the Central Valley by the U.S. Geological Survey and by the Lawrence Livermore National Laboratory, among others.

What is the average age of wells that are in exceedance of the MCL in the Salinas Valley study area?

We did not collect construction date data as part of this study. Nitrate levels in a well are independent of the age of the well.  The age of the well, however, may play a role in the construction of the well, especially the screen placement.  When available, we collected construction information including information on screen location. Technical Report 4 (Chapter 5) investigates the relationship between nitrate and well depth.

Isn’t nitrate from fertilizer that was applied many years ago causing the groundwater contamination problem we are experiencing today? If so, why should today’s farmers be responsible for cleaning up contamination that we didn’t create?

Nitrate currently found in wells has been recharged years or decades ago.  At the same time, agriculture and other sources continue to contribute nitrate to groundwater, often at very high levels.  Agriculture continues to be the largest contributor of groundwater nitrate. The report outlined several options for funding drinking water solutions as well as addressing the source problem. Under the Porter-Cologne Act, California's water quality law, current and past polluters (and landowners) can be held responsible for addressing water quality impacts.

Is your analysis of the nitrate contamination problem not biased when some wells are sampled often and others just once?

We compiled a large dataset of groundwater nitrate data from many different sources. Some sources frequently sample their wells:  public water supply wells reporting to the California Department of Public Health, for example, regularly sample and analyze their well water.  In contrast, other programs may only sample a well once:  some counties test domestic wells immediately after construction; some research programs (e.g., the SWRCB GAMA Domestic Well Program) focus on a single survey of a larger number of wells.  Also, some programs have a long history (public wells have been sampled for over 10 years), while others may be short-lived or are brand-new (e.g., the groundwater sampling program under the 2007 Dairy General Order).
We therefore investigated the dataset in multiple ways and report data at various levels. Technical Report 4 (Chapter 5) provides tables and figures that break down nitrate occurrence by sampling program, by region, and by time period, among others. The study accounts for the fact that some wells are sampled once, while others are sampled many times. We also take into account the fact that nitrate concentration is not "normal" distributed, and therefore special statitistical methods are required.

How do you account for the fact that some areas that are particularly polluted have a lot of wells, but other areas with no pollution have almost no wells? Even if you account for the fact that some wells are sampled once and others many times, doesn't this really skew your results?

We used a spatial declustering technique that was first implemented by the U.S. Geological Survey. Technical Report 4 (Chapter 5) explains the methodology we used and presents results obtained after the spatial declustering was performed. We divided the study area into five major groundwater regions. Each groundwater region was divided into equal areas, and for each equal area, we determined a representative nitrate value that also account for the generally skewed distribution of nitrate among wells. Our methodology is consistent with advanced techniques used by the U.S. Geological Survey and other researchers in the field.

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