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Aquifer Monitoring
Aquifer MonitoringThe Idaho State Department of Agriculture (ISDA) regularly monitors groundwater quality in the Spokane Valley-Rathdrum Prairie Aquifer in order to better understand agricultural impacts on regional groundwater quality. This aquifer serves approximately 50,000-100,000 people throughout parts of Idaho and Washington and was evaluated for nitrate and pesticide levels as part of this monitoring program. This study began as a five-year project in 1998 and selected at random a number of domestic wells in the area for regular sampling. Groundwater monitoring results indicate that low levels of the pesticides Dacthal (DCPA) and 2,4-Dichlorobenzoic Acid (2,4-DCBA) were detected in three of the sampled wells (7.5% of the sample set) during 1998. Follow-up sampling in 1999 on these three wells indicated that these pesticides were no longer present. Results of nitrate sampling indicate that levels are far below the EPA maximum contaminant level (MCL) of 10 mg/L (nitrogen as nitrate), but that in some years, approximately 5% of the sites had nitrate concentrations greater than or equal to 2.0 mg/L. Nitrate Results for ISDA Spokane Valley-Rathdrum
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| Concentration Range (mg/L) | Summer 1998 30 Wells | Summer 1999 30 Wells | Summer 2000 30 Wells |
Summer 2001 30 Wells |
| <MDL (0.050)* | 15 (37.5%) | 13 (35.1%) | 13 (35.1%) | 10 (27.8%) |
| MDL to < 2.0 | 23 (57.5%) | 22 (59.4%) | 22 (59.4%) | 24 (66.7%) |
| 2.0 to < 5.0 | 2 (5.0%) | 1(2.7%) | 2 (5.4%) | 1 (2.7%) |
| 5.0 to < 10.0 | 0 (0%) | 1(2.7%) | 1(2.7%) | 1 (2.7%) |
| >10.0 | 0 (0%) | 0 (0%) | 0 (0%) | 0 (0%) |
| Median Value | 0.111 mg/L | 0.97 mg/L | 0.11 mg/L | 0.175 mg/L |
| Maximum Value | 3.3 mg/L | 5.0 mg/L | 3.2 mg/L | 5.5 mg/L |
Pesticide Results for ISDA Spokane Valley-Rathdrum
Prairie Aquifer Regional Project 1998
| Pesticide Detects | % Number of Detects 30 Wells |
Range (?/L) | Health Standard (?/L) | University of Idaho Estimated Detection Limits (EDL) (?/L)** |
| Dacthal | 2 (5%) | 0.46-0.61 | 100 (RfD)*** | 0.016 |
| 2,4-DCBA | 1 (2.5%) | 0.12 | 0.1 (RfD)*** | 0.010 |