GROUND-WATER QUALITY MAPPING FOR THE VALLEY-FILL AQUIFER IN MORGAN VALLEY, WASATCH RANGE, NORTHEASTERN UTAH

Morgan Valley, like several other hinterland valleys in the Wasatch Range, is a rural area characterized by extensive agricultural activity and increasing population. The unconsolidated valley-fill aquifer is Morgan Valley’s most important source of drinking water. In cooperation with the Weber-Morgan District Health Department, we mapped water quality in the valley-fill aquifer with emphasis on nitrate and total-dissolved-solids concentrations. Water-well samples were collected and analyzed during spring 2004. We selected 52 water wells, without bias to land-use practice, to represent a widespread valley-wide distribution. We supplemented our database using chemistry results from the Utah Division of Drinking Water for 24 wells and one spring. Most of the sampled water wells are less than 150 feet deep. Water samples from 76 wells were analyzed for nutrients (nitrate, nitrite, ammonia, and phosphate); 59 of those wells were analyzed for general chemistry and dissolved metals. Of those, five were tested for organics (including pesticides) and two for radionuclides. Ten wells previously sampled and having relatively high (>4 mg/L) nitrate concentration were sampled for nitrogen isotopes.

Average nitrate concentration for water wells in the valley is 2.6 mg/L. Eighty-nine percent of the wells analyzed for nitrate yielded values less than 5 mg/L. Three percent of the water wells analyzed showed high nitrate values (>10 mg/L). Preliminary data indicate most high nitrate concentration areas are localized and in recharge areas. Possible sources of nitrate include fertilizer, feed lots, and septic-tank systems. No water wells tested for pesticides showed detectable concentrations. Three wells have arsenic concentrations that exceed the EPA’s 10 µg/L standard. Total-dissolved-solids concentration for water wells in the valley ranges from 92 to 1018 mg/L with an average of 437 mg/L. Total-dissolved-solids concentration for 89 percent of the wells is less than 500 mg/L. The widespread agriculture activity in Morgan Valley appears to have only a minor impact on the quality of ground water. The results of our study indicate the valley-fill aquifer contains mostly high-quality ground-water resources that warrant protection.