HYDROLOGIC OBSERVATIONS FROM BEDROCK MONITORING WELL NESTS
IN THE BIG SPRING BASIN

R.D. Rowden and R.D. Libra


Iowa Department of Natural Resources, Geological Survey Bureau,
Open File Report 90-1,1990, 27 p.

ABSTRACT


The Big Spring basin is a 103 sq. mi. (267 sq. km.) groundwater basin in Clayton County,Iowa. The groundwater discharge and water quality within the basin have been monitored continuously since 1981. Big Spring discharges from the Galena aquifer, the main groundwater source in the basin. It is a carbonate aquifer with moderate karst development. In previous studies, water table/potentiometric data from Galena wells and spring elevations, along with dye tracing and identification of losing- and gaining-stream reaches, were used to define the extent of the groundwater basin.

To refine the three-dimensional understanding of the groundwater-flow system, test holes were drilled at five sites. At four of these sites, cores were taken and nested monitoring wells were installed. Well sites were placed in different regions of the basin to delineate the hydrologic system, and to refine the potentiometric relations within the flow system of the Galena aquifer and bounding aquitards.

Site BS-1 is located in the southernmost portion of the basin near the Big Spring and the Turkey River. In this portion of the basin, the top of the Galena potentiometric surface declines to within 50 to 75 feet (15 to 23 m) of the base of the aquifer. The Turkey River acts as "base-level" for the Galena potentiometric surface. The bedrock dips toward this area and groundwater is discharged through Big Spring and associated smaller springs to the Turkey River. At this location, hydraulic head increases with depth, and the potential for groundwater movement is upward.

Site BS-2 is located near the center of the basin, above an inferred major conduit zone. In this portion of the basin, Robert's Creek and Silver Creek and their associated alluvial valleys are 100 feet (31 m) or more above the Galena potentiometric surface, which lies roughly in the middle of the aquifer. In this portion of the basin the potential for groundwater movement is downward. A complex system of small voids was encountered at this site.

Site BS-3 is located in the north-central portion of the basin near an area of sinkhole development. At this location, the Galena potentiometric surface was expected to be near or above the top of the Galena aquifer. Installation of the lower Galena observation well showed the potentiometric surface to be approximately 125 feet (38 m) below the top of the Galena at this location. This site may be over a conduit zone where the Galena potentiometric surface is locally depressed. The feature may not be large enough to have been reflected in previous assessments of water-level data. At this location, the potential for groundwater movement is also downward from the Galena to the St. Peter.

Site BS-4 is located in the northwest portion of the basin. In this area, Galena carbonates are overlain by the Maquoketa Formation, and the Galena potentiometric surface is located above the top of Galena. At this site, an anomalously thick sequence of Quaternary materials overlie an incomplete Maquoketa section. In this portion of the basin, the potential for groundwater movement is downward; the Quaternary and Maquoketa rocks form an aquitard, and the Galena is a confined aquifer.

For continuous monitoring of the wells, digital recorders were installed and set to register water levels hourly. From January 1989 to September 1989, all monitoring wells exhibited overall declines of mean monthly water levels, reflecting the continued absence of recharge during the drought. The more shallow wells exhibited more immediate response to snow melt and rain fall events than the deeper monitoring wells.

Water-quality sampling of the monitoring wells is expected to begin during the 1990 water year. Ongoing monitoring of water levels in wells will provide detailed three-dimensional observations of changes in the various potentiometric surfaces within the basin, and refine knowledge of potentiometric relations and contaminant transport within the flow system of the Galena aquifer and bounding aquitards.