INTENSIVE GROUNDWATER MONITORING: A GOOD THING? A SHALLOW SUBURBAN ALLUVIAL SOIL AQUIFER, NORTH CENTRAL OKLAHOMA, REVISITED

Characterizing natural groundwater quality in shallow aquifers for the purpose of comparison against potentially contaminated water opens a Pandora’s box of questions. Myriad physical, chemical, and biological processes impact groundwater chemistry and flow patterns in conflicting and reinforcing ways. Monitoring networks evolve as data needs change, conceptual models mature, and project budgets fluctuate. Scientists may become engaged in a public conversation to explain uncertainties and ambiguities that often result as a unintended byproduct of intensive data gathering efforts. Reexamination of historical data gathered from an intensively monitored field site in north central Oklahoma illustrates some of the issues involved in characterizing shallow alluvial aquifers.

The Ashport alluvial aquifer consists of 13.1 meters of surface soil and upper and lower buried paleosols overlying Permian shale in Payne County. Over a period of approximately four years, 41 wells, eight soil-water suction lysimeters, and four neutron access tubes were installed at the 2400-m² suburban site. Wells were typically installed in clusters of individual wells screened at discrete intervals between 2.0 m to 12.3 m below land surface. The site was monitored for precipitation, soil moisture content, water level, bulk water quality parameters, nitrate-N, chloride, bicarbonate, and major cations. Tracer tests and pumping well tests were performed to evaluate chemical transport parameters.

Horizontal hydraulic gradient and depth to the water table, two parameters integral to predicting groundwater transport, varied substantially over periods of months to days in response to local vegetative consumptive use and regional flow dynamics related to a nearby creek. Electrical conductivity of groundwater across the site varied from approximately 250 to 2000 micromhos/cm during one 11-month period. An order of magnitude change in nitrate-N was noted in one lysimeter following a recharge event. However, groundwater recharge and transport of chemicals to the aquifer during precipitation events depended on antecedent moisture conditions, so that amount of rainfall alone was not a good predictor of nitrate-N transport to the aquifer. These observations highlight the perennial challenge of characterizing alluvial aquifers.