Abstract

The City of Moroni is in northern Sanpete Valley, a rural agricultural region approximately 60 miles south of Salt Lake City. Moroni is located where the Sanpete Valley diverges into two branches, one extending toward the northeast and the other toward the northwest. Prior to August 1994, Moroni obtained municipal drinking water from a 300-foot deep well on the south side of town. Routine sampling in August 1994, revealed that nitrate levels in well water slightly exceeded the federal maximum contaminant level of 10 ppm. Furthermore, nitrate levels in the town’s back-up well were over 5 ppm and rising. Moroni thus decided to construct a new municipal water supply well. To be effective, the new well must yield at least 900 gallons per minute (gpm). Additionally, budgetary constraints make it highly desirable to locate the new well within one mile of Moroni’s existing water tanks.

We performed an analysis of available nitrate and hydrogeologic data, and identified two potential locations for a new well: one in the southern Cedar Hills north of Moroni, and one in the northwest branch of the Sanpete Valley northwest of Moroni. The bedrock in the southern Cedar Hills has generally low permeability, so a well with adequate yield could only be constructed within a zone of highly fractured rock. While our subsequent geologic investigation succeeded in locating an area that appears to have relatively high fracture density, other wells in the targeted area have low yields. We, therefore, recommended no development in this area without first drilling a test well to further assess the aquifer characteristics.

Because of the potential problems with adequate yield from a well in the Cedar Hills, we next assessed the potential for a well in the northwest Sanpete Valley. Regional well data indicate that a portion of the northwest Sanpete Valley is underlain by a thick layer of high-permeability material, and a well located in this area would likely yield an adequate supply of water. There are, however, documented occurrences of high nitrate concentration and several apparent nitrate sources upgradient of the targeted area in the valley. Due to the possibility of encountering high nitrate, we further assessed the hydrology and geology of the valley to locate a long-term, low-nitrate source of ground water.

Available data indicate that ground water within part of the northwest Sanpete Valley has historically had very low concentrations of nitrate. Data collected by Kleinfelder support the historic data, indicating that ground water within much of the northwest Sanpete Valley remains relatively free of nitrate, despite the apparent presence of upgradient sources. The low nitrate levels may be due to dilution of the ground water by upwelling clean water from an artesian system and/or the effect that the artesian flow may have on the ground-water flow regime. The northwest Sanpete Valley appears to be a viable location for a municipal well.