Introduction

In 1993 and 1994 the U.S.G.S. collected water samples from selected monitor wells, Fossil Gulch Canal and perched aquifer discharge springs to be analyzed for a suite of chemical constituents. Dissolved nitrogen (NO2+NO3) concentrations in all of the wells and springs tested are above the concentrations of the canal and Snake River water. Figure 47 illustrates the location and concentration of nitrogen in the monitor wells, Fossil Gulch Canal and the Snake River.

Results

Nitrogen (NO2+NO3) concentrations are elevated above the canal and Snake River water in all of the wells tested. Ground water nitrogen concentrations range from 3.0 to 39.0 mg/l (milligrams per liter) but most are in the range of 4.0 to 10.0 mg/l while the canal water tested at 1.4 mg/l (U.S.G.S., 1993). Snake River water samples were collected by the University of Idaho Water Resources Research Institute about five miles upstream of the study area and tested for nitrogen. Nitrogen (NO2+NO3) concentrations of the Snake River ranged from 0.75 to 1.98 mg/l and averaged about 1.46 mg/l (Brockway and Robison, 1992). This value corresponds with the U.S.G.S. canal water test of 1.4 mg/l.

Conclusions

Dissolved nitrogen concentrations from monitor wells are elevated above the canal and river water suggesting there are other sources of recharge to the perched aquifer systems besides the Fossil Gulch Canal. Generally, the lower the nitrogen concentrations in a particular well the better the hydrologic connection with the canal water. A mixing effect is probably taking place in some monitor wells due to long effective screen intervals. For example, water that has percolated down from agricultural crops (i.e. high in nitrate concentrations) may be perched at the base of the Tuana Gravel Formation. This water may be draining into lower perching systems (i.e. low in nitrate concentrations) through monitor well screen intervals and influencing nitrate concentrations.

Figure 47 nitrate map (omitted from on-line edition)

Table of Contents
Chapter 1 | 2 | 3 | 4 | 5
Appendix A | B | C | D

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