Protecting the Source of Your Drinking Water
by Keith Schilling and Mary Howes
Have you ever wondered where the water you drink comes from? In Iowa, chances are if you traced the water back through its distribution system, you would find a well pumping water out of the ground. But where does this groundwater come from? The question is important to public water suppliers who are entrusted with delivering good quality water to your home. Water utilities that can identify the source of their water can work to protect these areas from contamination and prevent potential contaminants that are spilled on the ground or released below ground from reaching their wells. As part of a new Department of Natural Resources effort called the Source Water Protection Program, the Geological Survey Bureau will be helping public drinking water suppliers over the next four years to determine protection areas for their water supply sources. States such as Iowa that receive Federal funds for water-supply improvement projects are mandated by the 1996 Federal Safe Drinking Water Act Amendments to conduct this source water delineation and assessment program.
The geological sources of groundwater are known as aquifers, and they vary greatly across the state in terms of their depth, horizontal extent, composition, and permeability. How are protection areas for wells pumping from these diverse sources determined? It often depends on the amount of existing information about the wells and the aquifers they tap. In the case where no information is available, it is easiest to select an arbitrary area, such as a 2,500-ft radius circle around a well. This arbitrary fixed-radius method then assumes that all water within 2,500 feet of a well constitutes its protected source. When information is available, a more accurate method for defining a protection area involves the concept of "time-of-travel " (TOT), or how far water travels in an aquifer during a specified period. For example, if water requires ten years to travel 2,600 feet, then a source water protection area, based on a 10-year TOT, would extend 2,600 feet from the well. For Iowas Source Water Protection Program, a two-year time-of-travel distance is the minimum recommended, but a ten-year distance is strongly encouraged. Ideally, a TOT should be selected that would allow adequate time for a water utility to respond to a contaminant release by cleaning up the source area or by replacing the water supply.
Time-of-travel, then, is the criterion the Geological Survey Bureau will use whenever possible to identify the area around a well that will become the focus of protection efforts. The most commonly used methods to delineate the geographic extent of this area include: 1) the calculated fixed-radius method; 2) hydrologic mapping; and 3) computer modeling. The value of each method depends on the quantity and quality of information available about the individual wells, the aquifer involved, and the hydrogeologic setting of the area.
The illustration above compares the results of each TOT method to determine a source water protection area for six wells installed in a shallow alluvial aquifer. A cross section through the valley (see line A - A on map, above) shows a three-dimensional view of this aquifer. Alluvial aquifers occupy river valleys and are typically composed of layers of sand and gravel deposited by flowing water. These aquifers are attractive to water suppliers because the shallow, highly permeable sand and gravel provides abundant groundwater that is relatively free of natural contaminants (e.g., hardness, iron). However, alluvial aquifers often lack thick overlying layers of low permeability materials to slow the recharge from above, and thus they are particularly vulnerable to contaminants introduced at the land surface.
The least complicated method for determining a source water protection area is a calculated fixed-radius, consisting of a circle drawn around a well to delineate a specified time-of-travel. The radius of the circle is defined by the wells pumping rate, the thickness and permeability of the aquifer, and the specified time-of-travel. For multiple wells, the calculated circles are merged to define the protection area for the entire wellfield. This method is relatively easy to perform with limited information, but it often results in a larger wellhead management area than other methods. In a narrow alluvial valley for example, this technique may unnecessarily include the surrounding uplands in the protection area.
A second method uses the physical and hydrologic characteristics of the area to map the protection zone. With this method, the protection area around a well is governed by the presence of groundwater flow boundaries which are based on rock and soil characteristics, the extent and thickness of the aquifer, and surface and groundwater drainage divides. In the example, boundaries are established to the east by lateral extent of the alluvium and low permeability of the valley walls, and on the west by a river that acts as a constant recharge boundary (constant supply of water). Because groundwater flow within alluvial aquifers usually parallels the direction of river flow, in this case northeast to southwest, the delineated protection area is primarily along the river. One difficulty in using this method is the accurate determination of the upstream and downstream boundaries that correspond to the selected TOT distance.
A third method of delineating a protection area relies on a computer program to model the groundwater flow system. Computer models typically require large inputs of data describing the dimensions and hydrogeologic properties of the aquifer materials in order to simulate the aquifers behavior. Information is also needed about well construction, pumping rates, river conductance (how much water flows through the base of the river channel), recharge and evapotranspiration rates. Models can be either two- or three-dimensional representations of groundwater systems. In our example, a three-dimensional model was used to simulate groundwater flow in response to pumping from six production wells. Water flow paths are determined in the model by placing "particles" at the wells and instructing the computer to trace their routes backward for the specified TOT, here ten years. The area bounded by the flow paths then becomes the source water protection area. The results show that little groundwater is drawn from the down-gradient side of the pumping wells (southwest) and that the protection area extends a considerable distance up-gradient to the northeast. Although the results of modeling often imply a high level of accuracy, one of the fundamental principles of groundwater modeling is that without sufficient data to properly formulate a model, the results may be no closer to reality than the arbitrary fixed-radius method.
The methods used to delineate source water protection areas
each require increased data to provide reliable estimates of
protection areas. The Geological Survey Bureau will select a
delineation method for each public water supply based on the
quality and quantity of hydrologic data that is available. Over
the next four years, protection areas will be determined for
approximately 1,950 public water supplies consisting of
approximately 5,300 wells. For many public supplies, the
arbitrary fixed-radius method will be sufficient. Much greater
accuracy will be required for more vulnerable water supplies.
Though a daunting task, identifying and protecting these areas
will ensure that high quality water will be available for
generations to come.
Adapted from Iowa Geology 1998, Iowa Department of Natural Resources