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GeoInsighter Summer/Fall
2003 Newsletter
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To many folks, summer means vacations, trips
to the beach, and outdoor barbeques. To the environmental professional,
summer means long field days, large field projects, and the rush to get
projects completed before the weather turns bad (especially here in New
England). However, for geologists, hydrogeologists, and environmental
engineers, summer has a much more important meaning – LOW WATER! That’s right, low water. In New England, the lack of rain during the summer months typically results in a lower water table during the months of July through September. In most geologic settings, the water table during the summer months is several feet lower than average, and in some settings, can be as much as tens of feet lower. It is not unusual for bedrock monitoring and supply wells to experience drops of up to 50 feet during the summer period. These changes in the water table provide an opportunity to obtain additional characterization information and often provide windows of opportunity to perform focused investigation or remedial activities. With regard to characterization, many regulatory agencies request that data be obtained to evaluate whether the direction of ground water flow changes appreciably over the course of a year in response to seasonal changes in the local water cycle. Summer conditions are typically considered to be representative of “stressed” conditions, and data obtained during the summer months (i.e., July through September) are often considered to represent “low water.” Summer is also a good time to examine ground water/surface water interactions. Many smaller streams dry up during the summer months or their flows reduce significantly. Having data to support that a stream (or wetland area) becomes dry during the summer months can be used to qualify the possible impacts to the local ecology and associated risk (i.e., intermittent streams do not typically represent significant exposure habitats because organisms would not be expected to be present year round). Changes in water level can also impact conditions within ground water plumes. Although conditions vary from site to site, generally, dissolved concentrations of volatile organic compounds (VOCs) in plumes increase in the summer because there is less water entering the system via precipitation (and thus, less dilution of the ground water). These effects are typically greatest in the shallow ground water system and can diminish with depth. Many regulatory agencies request that ground water quality data be obtained from periods of “wet” and “dry” conditions to evaluate whether VOC concentrations within plumes fluctuate appreciably in response to seasonal hydraulic conditions. These data can then be used to support the use of certain values to characterize potential risk (regulatory agencies are often concerned that data obtained during investigation activities are representative of “best case” conditions and that subsequent risk management decisions not be based upon limited data). With regard to remediation, low water conditions often make it easier and less expensive to perform soil removal activities. In particular, late summer is a good time to perform soil excavation activities that are focused upon removing residual petroleum impacts from what is typically referred to as the “ground water smear zone,” the zone where ground water fluctuates over the course of a year. By performing soil excavation activities during periods of low water, costs are typically reduced because there is less need for liquids handling and the soil to be removed generally weighs less (the moisture content of the soil is lower). Low water conditions also generally expose a greater area of vadose zone soil to the influence of soil vapor extraction systems. As such, summer is a good time to maximize the “pull” of such systems. It is also during the summer months that one typically sees the greatest recovery of separate phase petroleum. For many sites, this increase in separate phase recovery is related to the decreased viscosity of the petroleum caused by elevated surface and ground water temperatures. In some geologic settings, the lowering of the water table also increases the mobility of the separate phase petroleum and results in increased flow to recovery wells and trenches. So don’t be surprised if your local hydrogeologist calls you to say that they’ve scheduled a ground water monitoring event or remedial system checkup during the next few months. Now you’ll know why. And if they haven’t called yet, you may want to call them to see why they haven’t!
Michael J. Webster, P.G., L.S.P.
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