David Robert Lee

A Field Exercise on Groundwater Flow Using Seepage Meters and Mini-piezometers*

Basic principles of physical hydrogeology and the nature of the hydrologic interactions between groundwater and surface water can be convincingly demonstrated in the field using two inexpensive and...

Groundwater seepage as a nutrient source to a drainage lake; Lake Mendota, Wisconsin

Abstract The flux of groundwater through shallow-water sediments into Lake Mendota was calculated from hydrologic studies and was measured directly with seepage meters at 106 sites around the lake. Groundwater accounted for a substantial amount (around 30%) of the water budget. Pore water in seepage zones was collected for chemical analysis by two methods: (1) dialysis samplers; and (2) direct gentle suction. This pore water, which was assumed to represent seepage inflow, was considerably lower in nitrogen and phosphorus than surface inflow but was higher in phosphorus and lower in nitrogen than well water, indicating that well chemistry does not provide a good indication of the composition of groundwater entering lakes. Calculations indicated that seepage accounted for 12% of the total phosphorus loading to Lake Mendota and 2% of the total nitrogen budget. These results are interpreted in terms of the annual nutrient loading estimates that have been done on Lake Mendota involving only surface water measurements.

Method for locating sediment anomalies in lakebeds that can be caused by groundwater flow

Abstract A device was designed to be dragged along lakebeds for locating anomalies in sediment temperature or chemistry. The drag was a density-balanced probe containing temperature transducers and an electrical conductivity cell. The drag was used successfully to detect an artificially created seepage area in soft bottom sediment below 8 m of lake water. The method may find application in estuarine or shallow ocean environments where variations in sediment properties can be used to identify anomalies, e.g., zones of groundwater flux.

LOCATING GROUNDWATER DISCHARGE IN LARGE LAKES USING BOTTOM SEDIMENT ELECTRICAL CONDUCTIVITY MAPPING

Groundwater-surface water studies that use conventional near-shore piezometers and /or seepage meters are impractical in larger, areal extensive lakes, as they require exorbitant numbers of instruments to quantify groundwater discharge zones. In smaller lakes an electrical conductivity mapping method has proven useful in mapping groundwater discharge zones. The technique identifies groundwater discharge by measuring variations in sediment pore water electrical conductivity and reduces the number of instruments necessary to quantify inflow, thereby lowering instrumentation costs and increasing a studys efficiency. This study sought to determine the techniques applicability in larger lakes. Thus the method was tested within the Hamilton Harbour, at the western end of Lake Ontario. This study found systematic variations between nearshore and offshore sediments and identified three anomalous zones that were thought to represent groundwater inflow. Onshore and offshore piezometers were used to verify the presence of upward gradients and elevated electrical conductivities. The sediment probe survey provided qualitative maps of areas of elevated electrical conductivity indicative of groundwater discharge and allowed a fairly extensive shoreline to be mapped quickly and economically. Survey results guided the installation of nearshore piezometers to discharge zones, eliminating the inefficiency of more conventional “hit or miss” point source installation approaches. This research demonstrated that the sediment probe was a valuable tool for studying groundwater inputs into large lakes.

Installing Piezometers in Deepwater Sediments

A new method has been developed for installing piezometers in the sediments of deep harbors and lakes, where it has been difficult to measure hydrogeological parameters and collect pore waters for geochemical analyses. Using an underwater hammer operated from the surface, piezometers have been installed as much as 12 m below the water-sediment interface. The piezometer screen is held in place by barbs on the drive head and is connected to the water surface via flexible tubing. These piezometers have been installed from boats and from ice cover in water up to 30 m deep. However, installation in water more than 100 m deep should be possible.

Reactive transport modeling of 90Sr sorption in reactive sandpacks

Strontium-90 ((90)Sr) is one of the most problematic radioactive contaminants in groundwater at nuclear sites. Although (90)Sr is retarded relative to groundwater flow, it is sufficiently mobile and long-lived to require treatment in many hydrogeological settings. A detailed study was performed on the practicality of using granular clinoptilolite as a sandpack around groundwater wells where groundwater is contaminated with (90)Sr and the water table must be lowered. The effectiveness of the reactive sandpack concept and the mechanisms controlling (90)Sr attenuation was investigated by numerical analysis of data obtained from four in situ column experiments. The experiments spanned the range of pore-water velocities that would occur during radial flow through granular clinoptilolite sandpacks. A kinetic sorption model was required to adequately reproduce the experimentally observed (90)Sr behavior. Calibrated first-order kinetic rates were correlated with pore-water velocities. After calibration, three sorption models were used to simulate (90)Sr attenuation for four hypothetical pumping scenarios. Results show that a velocity-dependent kinetic model accurately simulates the observed early breakthrough for high pore-water velocities. The results indicate (1) that reactive sandpacks have good potential for in situ remediation and construction dewatering and (2) that quantitative modeling can aid in the design and application of this novel technique.

Longevity Estimates for a Permeable Reactive Barrier System Remediating a 90Sr Plume

A permeable reactive barrier system, known as the Wall and Curtain, was installed at the Chalk River Laboratories, Chalk River, Ontario, in 1998, to intercept a 90Sr plume. The system employs clinoptilolite, a zeolite, as a reactive material which adsorbs 90Sr. Reactive transport simulations of the site were conducted using the numerical code HydroGeoSphere to provide longevity estimates for the system. The HydroGeoSphere simulations included three solutes, for which zoned distribution coefficients were specified. Longevity estimates derived from the simulation were between 70 and 100 years for the Wall and Curtain system.