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Scientific Investigations Report 2006-5206

Prepared in cooperation with the Washoe County Department of Water Resources

Quantification of the Contribution of Nitrogen from Septic Tanks to Ground Water in Spanish Springs Valley, Nevada

By Michael R. Rosen, Christian Kropf (Washoe County Department of Water Resources), and Karen A. Thomas

Download the report as a PDF (8 Mb): SIR2006-5206

ABSTRACT

Analysis of total dissolved nitrogen concentrations from soil water samples collected within the soil zone under septic tank leach fields in Spanish Springs Valley, Nevada, shows a median concentration of approximately 44 milligrams per liter (mg/L) from more than 300 measurements taken from four septic tank systems. Using two simple mass balance calculations, the concentration of total dissolved nitrogen potentially reaching the ground-water table ranges from 25 to 29 mg/L. This indicates that approximately 29 to 32 metric tons of nitrogen enters the aquifer every year from natural recharge and from the 2,070 houses that use septic tanks in the densely populated portion of Spanish Springs Valley. Natural recharge contributes only 0.25 metric tons because the total dissolved nitrogen concentration of natural recharge was estimated to be low (0.8 mg/L). Although there are many uncertainties in this estimate, the sensitivity of these uncertainties to the calculated load is relatively small, indicating that these values likely are accurate to within an order of magnitude. The nitrogen load calculation will be used as an input function for a ground-water flow and transport model that will be used to test management options for controlling nitrogen contamination in the basin.

Contents

Abstract

Introduction

Description of the Study Area

Why was this Study Needed?

What was Measured?

How Were Nitrogen Loads Calculated?

How Did Nitrogen Concentrations Vary?

Estimates of Nitrogen Loss

How Much Nitrogen is Contributed by Septic Tanks?

Conclusions

What Future Work is Needed?

Acknowledgments

References Cited

Figures

Figure 1. Location map of the Spanish Springs Valley study area.

Figure 2. Location of lysimeters installed in the leach fields of the four locations used in this study.

Figure 3. Monthly total dissolved nitrogen measurements from individual lysimeters installed in the four septic tank leach fields.

Figure 4. Median total dissolved nitrogen concentrations for all shallow and all deep lysimeters sampled within the leach fields of the septic tank systems.

Tables

Table 1. Variables and values used for the Hantzsche and Finnemore (1992) model used to calculate loads.

Table 2. Average, range, and median total dissolved nitrogen concentrations (TN), one standard deviation (SD) of the average values, and samples counts for each sample site.

Send questions or comments about this report to the author, Michael R. Rosen

For more information about USGS activities in Nevada, visit the USGS Nevada Water Science Center home page.

This report is available online in Portable Document Format (PDF): SIR2006-5206

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