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<br />00112 t <br /> <br />. <br /> <br />. <br /> <br />. <br /> <br />the Mlay /Cook memo, the minimum water quality standards for recharge water were set at <br />the 1986 Amendments to Safe Drinking Water Act Primary DrinlGng Water Standards, No <br />exceedances were detected as a result of recharge activities, so no responses were needed, <br /> <br />The aquifer is hydrologically truncated by erosion 360 degrees around, Recharge is due <br />only to infiltration from rainfall and snowmelt on the Turner-Hogeland plateau. In the <br />project area, the water table is about 20 feet below land surface, and the aquifer is about <br />30 feet thick. The aquifer is underlaid by Bear Paw shale, which is considered to be <br />impermeable. Aquifer total dissolved solids (TDS) is <400 milligrams per liter (mg/L). <br />Aquifer vulnerability is considered high and susceptibility somewhat less high, <br /> <br />Baseline monitoring at this site showed that the ambient groundwater quality was generally <br />good, with an average TDS content of 365 mg/L. Nitrate-nitrogen was the only analyte <br />showing exceedances of the maximum contaminant level (MCL) for drinking water, <br /> <br />Before starting this project, the cooperators agreed that if some of the analytical parameters <br />were not detected, they could be dropped from the list of analytes, After baseline <br />monitoring, all of the identified analytes proved to be of no regulatory concern, After <br />detecting higher than expected nitrate-nitrogen concentrations in the groundwater, <br />additional monitoring was initiated. Nitrate-nitrogen samples were collected whenever <br />possible during the project's demonstration phase, There was no monitoring for micro- <br />biological parameters and no microscopic particulate analysis because it was assumed that <br />natural filtration would remove all microbial contamination infiltrating from the surface <br />and vadose zones, <br /> <br />For most water quality constituents, this project had very little effect. Statistical analysis of <br />a few constituents showed no impact at a 95 percent confidence interval. There were no <br />detections of organic compounds during the demonstration phase of this project, No <br />exceedances of drinking water standards (MCLs or health advisories) were found during <br />the demonstration phase of this project except for nitrate-nitrogen, Other inorganic <br />constituents were remarkably constant and not of regulatory concern, All other analyte <br />concentrations measured within the experimental plot were essentially unchanged from the <br />pre-project background conditions. <br /> <br />Nitrate-nitrogen concentrations clearly decreased during the project demonstration period, <br />Concentrations of nitrate-nitrogen at the beginning of the project ranged from a high of <br />greater than 22 mg/L to a low of less than 7 mg/L. Exceedances of the 10 mg/L nitrate as <br />nitrogen standard on a portion of the site were attributed to existing nutrients present in <br />the ambient groundwater prior to this project. Nitrification due to sod busting done as <br />preparation for this project could also contribute some nitrate to the groundwater, No data <br />were collected to confirm either hypothesis as the source of the nitrate contamination, At <br />the end of the project, nitrate-nitrogen ranged from less than 4 mg/L to just greater than <br />18 mg/L. No verifiable mechanism was demonstrated for this observation. Possible <br />mechanisms identified include dilution, reductive denitrification, or sorption to soil <br />materials, <br /> <br />5 <br />