THE EFFECT OF SEDIMENT DISTURBANCE ON EXCHANGE BETWEEN GROUNDWATER AND SURFACE WATER IN NEAR-SHORE MARGINS

Seepage across the sediment-water interface can be limited by low-permeability sediments situated at or near the bed surface, particularly in locations where wave energy and currents are minimal. Intentional disturbance of the sediment bed substantially increased seepage in the shallow, sandy, near-shore margins of two lakes where flow was from surface water to groundwater. Seepage was measured before and after walking on lakebed sediment at Mirror Lake, New Hampshire. Seepage increased by factors of 2.7 to 7.7 at 7 of 8 measurement locations where the sediment causing the greatest restriction to flow was situated at the sediment-water interface. Although the veneer of low-permeability sediment was very thin and easily disturbed, accumulation on the bed surface was aided by a physical setting that minimized current and wind-generated waves. At Lake Belle Taine, Minnesota, where pre-disturbance downward seepage was smaller than at Mirror Lake but hydraulic gradients were very large, disturbance of a 20- to 30-cm-thick medium sand layer resulted in increases in seepage of 2 to 3 orders of magnitude. Exceptionally large seepage rates, some exceeding 25,000 cm/d, were recorded following bed disturbance. Seepage meters were used to quantify flow across the sediment-water interface at both locations. Since it is common practice to walk on the bed while installing or making seepage measurements, disruption of natural seepage rates may be a common occurrence in near-shore seepage studies. It is recommended that disturbance of the bed be avoided or minimized when using seepage meters in shallow, near-shore settings where waves and currents are infrequent or minimal.