I mentioned in my “Practicality of Geology” post that one needs an understanding of geology in order to properly construct a water well that minimizes contaminants and avoids environmental disasters. Here is an example of how water well design is important for reducing the spread of pollution
Last week I attended a public awareness event hosted by the Environmental Protection Agency (EPA) regarding a groundwater contaminant plume spreading in under the western suburbs of the Phoenix metro area. The primary contaminant is trichloroethylene (TCE), an industrial solvent once commonly used in the aircraft and electronics industries. Over several decades, several manufacturing companies in the area dumped waste TCE into dry wells. The solvent migrated downward to the aquifer, and subsequently spread within the groundwater.
First, some basic definitions:
An aquifer is a unit of saturated permeable rock or sediment that can yield water to a well or spring. In simple terms, the “surface” of the aquifer is called the water table (the true definition of “water table” is slightly more complicated than that). Any saturated rock or sediment unit that does not easily yield water to a well is called an aquitard or “confining layer”. These confining layers are typically composed of finer sediment such as silt and clay.
The Phoenix area aquifer:
The large, thick aquifer beneath the Phoenix metro area is formed from layers of unconsolidated sediment deposited in a deep bedrock basin. The sediment alternates between layers of sand and gravel, and finer layers of silt and clay. The sand and gravel layers serve as the aquifer, whereas the silt and clay layers are aquitards that divide the aquifer into distinct layers. The aquitards prevent significant mixing of water between aquifer layers above and below the aquitard.
In the subsurface of the Phoenix metro area, geologists identify three significant layers. The “A” layer is made up of unconsolidated (meaning loose sediment instead of solid rock) sand and gravel and is saturated with water in its lower portions. The top of the A layer is the ground surface. Below the A layer, the “B” layer is composed of unconsolidated clay and silt and restricts the downward movement of water from the A layer. The “C” layer lies underneath the B layer. The sand, silt, and gravel of the C layer generally contain cleaner water because it is protected from surface contaminants by the confining B layer. Most area municipal drinking water wells draw their water from the C layer.
How wells spread pollution:
The former agriculture fields west of Phoenix were once irrigated by thousands of wells. Drillers of these agricultural wells only cared about how much water they produced. Drillers gave little care to water quality or sealing around well casings. The problem is, many of these wells are screened (or perforated) from above the water table continuously to the bottom of the well. Because the well screen crosses the B layer aquitard, contaminants in the upper A level migrate to the C layer using the well as a conduit. Since the C layer is where most nearby municipal wells draw their water, these wells and the people dependent upon their water are now at risk.
How science solves this problem:
Cleanup first requires containment, and that means drilling dozens of monitor wells. Monitor wells do not typically pump water for use, rather they collect data for understanding the dimensions and spread of the contaminant. Cleanup also means altering the water table so that the contaminant flows back towards the source, instead of spreading to uncontaminated areas. Another key cleanup requirement necessitates sealing the old agriculture wells with cement and clay, and regulating new wells to prevent further contamination between the A an C layers. Some wells are sealed only at critical depths using liners installed within the well casing. Cleanup of these groundwater contaminants is already a decades-old process, and will probably take decades more before the problem is mitigated.
For new wells, geologists examine the texture and composition of the aquifer materials as a means of understanding aquifer characteristics. This information reveals what layers are aquifers and what layers are aquitards. Monitoring water quality at specific depths during well drilling identifies the layers that have good and bad water. Other instruments and testing methods reveal more information about aquifer properties and water quality at various depths beneath the ground. When these data are combined, a well is designed that prevents cross contamination issues between aquifer layers. The design accomplishes this by not screening the well above the water table, through layers of poor water, or through confining layers. The annulus (area between the edge of the borehole and the well casing) is sealed with cement or clay below areas of bad water in order to seal the bad water from the good water. Without these measures, wells enable the spread of pollution through the aquifer.
The EPA webpage for this specific superfund site is found here. While the contaminant in this case is a human pollutant, naturally undesirable water water (such as water containing salt, hardness, and arsenic) often spreads in the same fashion through poor well construction practices. Unfortunately, many people do not consider this issue serious, and continue drilling wells without regard to any other criteria except water quantity. A single well many miles away can permanently affect the groundwater quality beneath where many people live. In places where individual households maintain private wells, this situation is especially worrisome. Educating the public to these potential hazards, and proper regulation of polluters, well drillers, and groundwater users is key to avoiding this costly health risk.
(PS – please excuse my crude power point drawings)