Water-Quality Analyses of Spanish Springs Wells

Nitrate concentrations in municipal water supply wells in Spanish Springs Valley have increased over the last fifteen years of monitoring (figure 1) and have been attributed to the increasing use of septic systems in the valley over this time.  Rosen and others (2006) have shown that over 30 metric tons per year of nitrogen are currently contributed to the aquifer by septic systems.  Due to the increasing nitrate concentrations in this sole-source aquifer, in 1995 the Nevada Division of Environmental Protection (NDEP) issued a ruling requiring Washoe County to ensure all new housing is connected to the sewer system.  In April 2000, NDEP issued a directive to commence with preparation of a plan to connect existing houses to the sewer system.

A project is now underway to quantify the present ground-water nitrate contamination in Spanish Springs Valley, Nevada, and to develop a nitrogen budget and contaminant transport model for the area. The joint project is being carried out by Desert Research Institute, USGS, University of Nevada, Reno, and Washoe County Department of Water Resources. Samples for chemical (nutrient, wastewater contaminants, metals, and certain major ions), isotopic (nitrogen, oxygen, tritium) and age dating (CFC) analyses by the USGS National Water Quality Laboratory have been collected by Washoe County from approximately 50 wells within Spanish Springs Valley.  Interpretation of these data is needed to provide understanding for development of contaminant transport models that can be used for management and prediction of contaminant movement.  Current work by USGS personnel into other sources of nitrogen to the ground water make USGS involvement in this overall interpretation of the data critical to the successful evaluation of the data.

Figure 1.  Nitrate concentration with time in a Spanish Springs water supply well.  Data from Washoe County Division of Water Resources.

Objectives

Provide interpretation of water-quality data collected from ground water monitoring wells and public supplies in Spanish Springs Valley that can be integrated into contaminant flow models and used to provide management options for Washoe County and provide oversight for Washoe County in the development of a feasibility report being written by the Desert Research institute that will provide management scenarios for Washoe County.

Strategy and Approach

Data will be interpreted using graphical, statistical and geochemical approaches.  Factors to be included are denitrification, dilution and travel times of contaminants.  CFC age dating will be interpreted along with tritium age dates to ensure a more reliable determination of modern water components in the aquifer. Noble gas data will be used to determine more accurate recharge temperatures, and to determine the possible role that denitrification plays in reducing nitrate concentrations in the aquifer. Correlation and multivariate statistical analyses will be used where appropriate. Logistic regression may be useful for determining likely areas where nitrate may be greater than the drinking water standard. However, an initial evaluation is necessary to determine if this is needed, as we already know the dominant source of nitrogen. Data from vadose zone studies and previous ground water work in the area and in Douglas County will be used to better constrain sources and flux of contaminants.

Relevance and Benefits

Interpretation of the ground-water quality data will provide water managers with an understanding of water movement and lag times that exist in Spanish Springs Valley.  In addition, sources and fate (for example denitrification) of nitrate and other contaminants will be interpreted and will provide information that can be used in predictive models and management scenarios. The proposed study is relevant to human- and environmental-health related issues identified in the Science Plan of the USGS Nevada Water Science Center, and to Issue 1 of the USGS Water Resources Discipline Strategic Directions which addresses the effects of urbanization on water resources. This study also will help the National Water Quality Assessment (NAWQA) program meet its goals of describing trends in the quality of the Nation’s water resources and describing factors related to those trends.

Publications

Kropf, C.A., Dragan, D., and Rosen, M.R., 2006, Nitrate contamination of municipal water supply wells: investigating from source to receptor: Groundwater Resources Association of California Symposium, Nitrate in California's Groundwater:  Are We Making Progress?, Modesto, Calif., April 4-5, 2006

Rosen, M.R., Kropf, Christian, and Thomas, K.A., 2006, Quantification of the contribution of nitrogen from septic tanks to ground water in Spanish Springs Valley, Nevada: U.S. Geological Survey Scientific Investigations Report 2006-5206, 12 p.

Rosen, M.R., Thomas, K.A., and Kropf, C.A., 2006, Quantification of the contribution of nitrogen from septic tanks to ground water in Spanish Springs Valley, Nevada: Nevada Water Resources Association Annual Conference, Mesquite, Nev., February 21-23, 2006.

Kropf, C.A., Dragan, D., and Rosen, M.R., 2005, Septic nitrate contamination of municipal water supply wells in Spanish Springs Valley, Nevada: Nevada Water Resources Association Annual Conference, Reno, Nev., February 1-3, 2004.

Rosen, M.R., Kropf, C.A., Dragan, D., Tyler, S., and Shipley, D. O., 2004, Assessment of source strength from septic tank nitrogen contamination to Spanish Springs ground water: Nevada Water Resources Association Annual Conference, Reno, Nev., February 1-3, 2004.

Kropf, C.A., Dragan, D., and Rosen, M.R., 2004, Septic nitrate contamination of municipal water supply wells—trends and remedial efforts: Geological Society of America, Abstract with Programs, Vol. 36, no. 5, p 330.