GROUND-WATER SENSITIVITY AND VULNERABILITY MAPS FOR BASIN-FILL AQUIFERS, NORTHERN AND CENTRAL WASATCH FRONT, UTAH

Contamination from pesticides potentially poses a threat to human health and the environment along the northern and central Wasatch Front, Utah, where ground water from basin-fill aquifers is the primary source of drinking water. Using GIS technology and readily available GIS data, we mapped ground-water sensitivity and vulnerability to pesticides for these basin-fill aquifers to determine the extent to which pesticides pose a threat. This mapping was performed as part of U.S. Environmental Protection Agency-required management plans for four agricultural pesticides (alachlor, atrazine, metolachlor, and simazine).

Sensitivity to pesticides was determined by assessing natural factors favorable or unfavorable to the degradation of ground water by pesticides applied or spilled on the land surface. We selected hydrogeologic setting (based on the presence or absence of confining layers and vertical ground-water gradient direction), soil hydraulic conductivity, retardation of pesticides, attenuation of pesticides, and depth to ground water as the five factors primarily determining ground-water sensitivity to pesticides. Ground-water vulnerability to pesticides was determined by assessing the effects of human-induced factors on our determined ground-water sensitivity to pesticides; the selected factors are the presence of applied water (irrigation) and crop type (corn or sorghum versus other crop types).

We compiled GIS maps by applying a combined process-based and index-based model, using the sum of numerical values assigned to the factors listed above, to produce low, moderate, and high sensitivity and vulnerability categories. Much of the northern and central Wasatch Front has low ground-water sensitivity to pesticides due to prevalent protective clay layers and upward vertical ground-water gradients within the basin-fill deposits. Areas of high vulnerability are located primarily in irrigated areas where ground-water sensitivity to pesticides is high. However, because of relatively high retardation (long travel times of pesticides in the vadose zone) and attenuation (short half-lives) of pesticides in the soil environment, pesticides applied to fields in the northern and central Wasatch Front likely do not present a serious threat to ground-water quality.