HYDROLOGICAL MASS BALANCE BETWEEN BURIED VALLEY SEDIMENTS AND BEDROCK AQUIFERS USING NUMERICAL GROUNDWATER MODELING

A buried valley is an ancient river or stream valley that predates the recent glaciation and since has been filled with glacial till and/or outwash. Outwash deposits are known to store and transmit large amounts of ground water. Their productivity largely depends on their hydraulic properties, rate of recharge and their hydraulic relationship with the adjacent bedrock formations. We illustrate these relationships by using MODFLOW to simulate steady-state three-dimensional flow through a section of a buried valley in Northeastern Ohio. The flow domain was divided into five hydrostratigraphic units: low conductivity (K) till and high K outwash within the buried valley and three bedrock units: Pottsville Formation, Cuyahoga Group and Berea Sandstone, while the pre-Berea formations served as a no-flow boundary. Model input data, e.g. spatial distribution of the hydrostratigraphic units and their hydraulic properties were estimated using the data from Well Log and Drilling Reports of residential water wells. The model was calibrated using observed hydraulic heads and flow data with mean residual head error of 0.3m (1.0’). The calibrated model was used to quantify flux between buried valley and bedrock formations. Mass balance for the model consists of 2.74 Mm³/yr ≈ 85% of inflow by recharge from precipitation and 0.48 Mm³/yr ≈ 15% of inflow across the upstream model boundary. Within the model, buried valley receives 1.6 Mm³/yr from the adjacent bedrock aquifers. Mass balance along the buried valley/bedrock contact indicates that Pottsville Formation contributes 0.96 Mm³/yr (or 0.34 Mm³/yr/km) while the Berea Sandstone contributes 0.64 Mm³/yr (or 0.22 Mm³/yr/km). Model outflow consists of baseflow 0.715 Mm³/yr drain, 2.88 Mm³/yr discharge at the lower boundary and 0.66 Mm³/yr outflow along a section of buried valley/bedrock interface. The calculated mass balance error is -2.2%.

Groundwater mass balance for the buried valley sediments
		Inflows		Outflows
Units		m3/d	%	m3/d	%
Drain (baseflow)		0	0	-1960	-17
Flow across the model boundaries		1015	9	-7880	-68
Recharge		5975	52	0	0
Bedrock aquifers	Pottsville Formation	2650	23	-1800	-15
	Berea Sandstone	1750	15
Total Flow		11390	100	-11640	100
Inflow - Outflow		-250			-2.2