High Yield Wells

Water Prospecting for High Yield Wells
The need for large yields of water requires an assortment of tools for the hydrogeologist, i.e. fracture trace analysis, detailed site reconnaissance, an understanding of the local geology and soils, geophysical surveys and experience. These methods greatly enhance the potential for success. However, you can employ all of the sophisticated groundwater prospecting methods in the world but there is no way to know how much water a certain well will produce until it is drilled, developed and pump tested. It would be disingenuous to imply otherwise.

Case Study
The following case study is of a residential gated-community development in the central piedmont of Virginia where supplemental water was needed for water features proximal to the existing residential development. Some of the elements of the job have been changed in order to protect the confidentiality and identity of my client. However, the results are accurate.

Before the client contacted True North, approximately seven (7) randomly located, groundwater wells had been drilled to find an adequate supply of water. Unfortunately, none of the wells could provide them with the desired volumes (frankly, only one was of any use).

As costly as this was, it is not an unusual situation. Most people feel that to hire a geologist prior to drilling is throwing money away. Consequently, people hire me after they have spent large sums of money drilling wells. This mistake is becoming less common as people learn more about how groundwater behaves.

The Steps Involved
The bedrock In Virginia is often obscured by vegetation and a deep mantle of weathered residuum (soil and saprolite). Regardless, there are resources and methods available in order to determine the geology of a specific location. The picture on the left is scanned from the Geologic Map of the Fredericksburg 30' X 60' Quadrangle by the USGS (a great map, by the way). This map is put together by extrapolating between bedrock outcrops that appear sporadically across large distances. Geology in the east is, in part, an art form because the scientist must make decisions based upon disparate pieces of information related to bedrock occurrence, changes in slope, geomorphology and soil.

These maps assist in determining the "big-picture" for a given site. But I also find it useful to obtain a copy of the county soil map and interpret the geology via soil association. This helps me to increase the geologic resolution for my projects.

The figure to the left is a soil map obtained from a local geographical information system (GIS) office that has been superimposed onto an air photo. In the example shown here I discovered that the following three distinct geologic units were present on this 200-acre site before I set foot on the property: a meta-granite, a meta-basalt and a meta-arkosic sandstone. This is useful information to a geologist in search of water and could have never been gleaned from the large-scale state geologic map.

Other resources examined include topographic maps and geotechnical reports of the site. It is also useful to know the average yields of existing wells in the vicinity of the site. While this information is not easy to obtain, it is helpful in evaluating the potential for a given locality.

The next step in the case study sited here was to conduct a fracture trace analysis to determine the occurrence and orientation of apparent fractures that exist on the site and site vicinity. The photo below is an example of suspected fractures having been superimposed upon an aerial photograph. Once this information was collected then a field trip was conducted in order to "ground-truth" fractures, find geologic outcrops (where possible) and record information (on the outcrop scale) related to fracturing and jointing. All of this information was evaluated to determine potential groundwater zones. Once the potential groundwater zones were exposed then True North made a judgment regarding the best placement for high-resolution resistivity surveys (in conjunction with the desires of my client). This part of the process is truly a team effort. The client must be given the best information possible and a decision must be made regarding the location of the groundwater source. This is where good communication is vital.

Based upon the fieldwork and background gathering, I was attracted to a certain region of the site (which my clients approved of) and placed our first geophysical survey to take advantage of it (seen below). The survey extended approximately 450 meters across the landscape and collected resistivity values to an approximate depth of 100 meters. The theory in this form of geophysics is that the lower resistivity values are associated with water bearing zones (its that simple, or so some would have you think). In general, the yellow, brown, orange and red colors suggest more resistant materials. The blue colors correspond to lower resistance, or higher conductivity (since conductivity is the inverse of resistivity) values.

The example shown above is an interesting case that underscores my philosophy regarding these computer-generated profiles. If the background-gathering phase of work had not been conducted and you had simply run the geophysics one might have been tempted to drill a well at 96 meters (see above) because of the lower values (lots of blue). I decided to drill a well at the 240 meter location because it backed-up what I had observed on air photos and what I saw in the field, even though the pretty picture generated by the computer beckoned me to drill somewhere else. It would be interesting to know if a large volume of water was present at the 96-meter mark; however, my clients pay me to find water and not necessarily to advance the understanding of hydrogeology or geophysics. More than anything, they want me to be right the first time.

Geophysical surveys are good tools but they aren't the gospel truth. They are important and very useful and are here to stay but they must be used judiciously and interpreted along with all of the available information. Nothing takes the place of good old-fashioned boot heel geology and the experience that comes with it.

The photo on the left is from the well drilled at the 240-meter mark on the geophysical profile above. As you might be able to tell, the well drillers were impressed with the 150 gallons per minute (as was my client). The well was drilled to a total depth of 300 feet (because we didn't need to go any deeper) and encountered three (3) significant water-bearing zones.