SIMPLE TOOLS FOR IDENTIFYING AREAS OF GROUNDWATER DISCHARGE AND QUANTIFYING DISCHARGE RATES

PushPoint and SedPoint porewater sampling tools (MHEproducts.com) can rapidly and accurately measure the hydraulic head of sediment pore water relative to the surface water elevation. The PushPoint is a ¼” diameter stainless steel probe with a discrete (1.75”) screened zone, and SedPoints are ¼” diameter polyethylene tubing microwells with a discrete (2”) polypropylene mesh screened zone. If either of these tools is installed in the sediments and connected to a manometer shape of vinyl tubing, the measurement is very simple. Porewater is pulled through tool and the manometer tubing, and then allowed to fall to its static elevation in the tubing. The water level in the tubing, above or below the surface water, represents the differential pressure between the pore water and surface water. If the meniscus is above the surface water then porewater is discharging to surface water, and if the meniscus is below the surface water then surface water is recharging groundwater at that location. Mapping these differential measurements across a sediment site accurately defines areas of groundwater discharge (and recharge) within that area.

These same tools can be used to quantify the bulk porewater discharge velocity in sediments. A method is being refined that utilizes micro-injections of dye through the screened zone of either sampler to instantaneously create a small dye slug at a specified depth within the sediments. This dye slug migrates to the sediment surface at the rate of porewater discharge, and the time from injection to expression is easily measured with a stopwatch or time-lapse photography. Where the dye emerges relative to the injection location provides information of how groundwater flowlines intersect at the discharge area. The method utilizes a sampler, a 60ml syringe, a four-way valve, and a few milligrams of a concentrated dye (such as fluorescein) in a 1ml syringe. Field testing has demonstrated that the procedure produces reproducible, and in some cases remarkable results. The discharge velocity, combined with measurements of the differential hydraulic head pressure and estimates (or measurements) of the sediment hydraulic conductivity allow the calculation of porewater flux for a given area. This method works very well in flowing water environments where other methods have difficulty.