Geochemistry of artificial-recharge tests in the Oakes aquifer near Oakes, southeastern North Dakota

As part of an artificial-recharge feasibility study, water from the James River was introduced into the Oakes aquifer of southeastern North Dakota by infiltration through a recharge basin. Chemical composition of water in the recharge basin and ground water from two separate flow paths beneath the basin was determined from samples taken during two artificial-recharge tests.

Changes between recharge-basin water and ground water from the termination of the flow paths during the spring 1987 test included increases in alkalinity, calcium, sodium, and silica and decreases in pH and dissolved oxygen. Interpretation of processes modifying the chemical composition of recharge water was complicated by mixing of recharge water with pre-existing ground water within the Oakes aquifer.

In the summer 1987 test, the recharge basin was lined with a decomposing organic mat. The purpose of the organic mat was to sustain large infiltration rates for a longer period of time and to evaluate the effect of a decomposing organic mat on water quality during sustained recharge-basin operation. Larger infiltration rates with the organic mat in place allowed recharge water to reach the termination of the flow paths prior to apparent mixing with pre-existing ground water for a period of as much as approximately 405 hours of recharge-basin operation.

Changes between recharge-basin water and ground water from the termination of the flow paths during the summer 1987 organic-mat test included increases in alkalinity, calcium, and silica and decreases in pH, dissolved oxygen, and total organic carbon. Detected changes between recharge-basin water and ground water from the termination of the flow paths were interpreted using a mass-balance geochemical model. Chemical changes of at least 2 milligrams per liter were modeled in terms of reactions, including those due to respiration of micro-organisms, dissolution of carbon dioxide, possible production of organic acids, dissolution of amorphous silica, cation exchange, and dissolution of carbonate minerals. Generation of organic acids within the decomposing organic mat may cause dissolution of dolomite in the subsurface.