Module 5: Groundwater Flow to Wells G & H
- Provide students with an overview of groundwater flow wells, formation of a cone of depression, superposition of multiple cones of depression, and creation of groundwater flow divides between wells.
- Let students discover the role of the Aberjona River as a potential source of water to wells G and H, and the role of the bedrock valley walls in limiting groundwater flow to wells G and H and shapes of their cones of depression.
One of the important questions raised during the trial centered on the drawdown of the water table when wells G and H were operating and the effects of the Riley tannery well. If, when wells G and/or H operated, the water table was drawn down on the west side of the Aberjona River, then contaminants from west side of the river valley (Beatrice 15 acres and Olympia Trucking) could flow under the river to wells G and H. If the water table was not drawn down on the west side of the river when wells G and H operated, then it is unlikely that contaminants on the west side of the river valley could flow to wells G and H. Thus, the interaction of these three wells with the Aberjona River and the groundwater flow system was critical to determining which of the contaminated properties contributed TCE and PCE to wells G and H.
Understanding groundwater flow to wells
The 30-day pumping test performed by the U.S. Geological Survey in December 1985 through early January 1986 was the first time wells G and H operated since their closure in May 1979. It was also the first time synoptic sets of water levels had ever been measured in the network of monitoring wells surrounding wells G and H. During the 30-day pumping test, wells G and H operated at their average rates of 700 gpm (well G) and 400 gpm (well H). However, pumping records kept by the City of Woburn showed that between 1964 and 1979 the wells rarely operated together and that the two municipal wells were frequently not in use for months at a time. As a result of changes in the pumping rates of wells G and H and their periodic (discontinuous) use, the water table in the buried valley aquifer was dynamic, rising and falling as wells G and H were turned on and off and pumped heavily or lightly. These dynamic changes in water levels result in the groundwater flow system being transient in character -- as opposed to being steady state in character.
Under steady-state conditions, water levels at any location in the flow system do not change with time, which results in hydraulic gradients and flow velocities at any location not changing with time, and therein no net change in the amount of groundwater in storage. Under transient conditions, water levels at any location in the flow system change with time, which causes hydraulic gradients, flow velocities, and the amount of groundwater in storage (water levels) to change with time. In this module we assume that the wells G and H have been pumping together for a long enough time that the water table is stationary and flow is steady state. This is an assumption that we recognize is not true over periods of time greater than a few months. This simplistic assumption, let's us learn about drawdown created by a single pumping well, the formation of a cone of depression, superposition of drawdowns from one or more pumping wells, and the creation of groundwater divides between pumping wells, all of which are important to understanding the responses of the groundwater flow system to the pumping history of wells G and H. The operation of the Riley well was estimated by de Lima and Olimpio (USGS, 1989) to operate at an average pumping rate of 200 gallons per minute, continuously. This was based on the wastewater discharge emanating from the tannery, actual pumping rates were not available. It is believed that pumping volumes fluctuated depending on the number of shifts working, the volume of leather orders, actual type of weather being processed, etc. USGS Circular 1139 provides an excellent overview of the relation between surface water, groundwater, and the sustainable use of these resources.