Groundwater Supply & Dewatering

Groundwater Supply & Dewatering 2017-12-13T12:07:55+00:00

water well pumpThe development and maintenance of a water supply are of importance to many people today, especially if one resides in the suburbs or in a rural setting. In large cities, water usually is supplied by a network of underground pipes from either a surface reservoir or from a system of high-capacity water wells. As population centers have expanded over the years, water wells often must co-exist with nearby oil and gas wells.

The selection process of deciding between a surface water or groundwater source depends on many factors:

  • geographic location (topography, climate, precipitation, temperature, and population density),
  • hydrogeologic conditions, (depth to water table and aquifers, suitability of aquifers, water quantity and quality,
  • engineering controls (cost sensitivity, political preferences), and contamination issues (surface-water vulnerability, groundwater vulnerability, and operator responsibility).

Litigation can result from disagreements between the landowner and well contractor, the municipal utility district (MUDs) and the operator and/or residents, or with insurance companies over a number of issues, i.e., water costs, water quality, causes of well failures, etc. Any of the above issues can be involved in litigation. Inappropriate or misrepresented data or methods are, in many instances, the basis of litigation involving activities conducted within the discipline involving groundwater supplies, which is practiced in some cases by unqualified individuals not having appropriate training and experience and the associated professional hydrogeological certifications and/or state licenses.

The Geographic Factors

In regions where the land surface is hilly or has some relief, a surface source of water can be developed by damming a river or large stream, or by sculpturing a reservoir out of lowlands that receive regular runoff. These projects are usually large engineering efforts that require large capital expenditures and a multidisciplinary team involving the U.S. Corp. Engineers, state and federal wildlife regulatory agencies, and local groups, some for and some against the project.

The political issues, which involve a large population, are often major but usually surmountable if the project makes sense to the majority of those interest groups involved. The projects are promoted on the basis of the multiple use of the land involved. The reservoir would not only hold a supply of water but also would be used for fishing, boating, swimming, as well as a focal point for surrounding developing residential subdivisions.

In the Texas and Oklahoma Panhandle, groundwater is used for irrigating crops. Since before the 1960s, withdrawals have exceeded recharge and water levels have dropped as a result. Only a few years now remain until water supplies are exhausted and the practice of irrigated farming will have to cease. Unless enlightened management intervenes, the Texas Panhandle area, now so dependent on groundwater, will soon be without a viable water supply to maintain the present way of life in the region.

The Hydrogeologic Factors

Groundwater supplies are usually available everywhere. Regional variation in water quality depends on the local makeup of the subsurface aquifer produced as a water supply. In high rainfall areas, the depth to water will be minimal, usually less than 20 feet below surface, depending upon the time of the year. The top of the groundwater reservoir is known as the water table. It varies over the year and adjusts to infiltrating local rainfall. During droughts, the water table declines. During years when rainfall is above normal, the water table will rise, sometimes high enough to create a temporary bog or swampy area. If this has happened on a regular basis over the years, a wetland may have developed.

For all practical matters, the water table should be considered a dynamic surface. The reason residents’ houses do not have basements in the Gulf-Coast area is because the rainfall is high and therefore the water table is too shallow to allow for the construction of basements. If constructed, the water would soon erode the cement slab making for rather wet conditions in the basement, even with the best cinder-block sealant. Without basements, and the attendant protection provided, residents become even more vulnerable to tornadoes than residents living in areas with less rainfall, lower water tables, and basements.

Groundwater taste also varies regionally. Water supplies that develop ground water usually involve minimal engineering, and usually involve a relatively small population base (see Campbell, et al., 1973). This source providing water for small groups of residents ranges from a few residents in small subdivisions to thousands of residents in the larger suburbs surrounding major cities and towns. Some large municipalities also utilize groundwater as opposed to surface water. These projects involve only minimal real estate, few permits, and generally little interference from state and federal agencies and polarized interest groups.

The Engineering Control Factors

The difference in cost between surface water and groundwater sources is substantial, for obvious reasons. Surface-water sources require large expenditures groundwater sources require small expenditures. Based on the cost per 1000 gallons of water delivered, surface-water costs run in the range of $0.85 to $1.35/1000 gallons, while many groundwater sources range from $0.30 to $0.80/1000 gallons (not including unusual treatment costs for special problems or other cost loading issues). It also should be noted here that the surface-water cost does not account for the other, all important benefits provided by the presence of a surface-water reservoir, i.e., fishing (less the license, bait and tackle costs), boating (less the jetty fees paid, license, and fuel costs).

The water-quality issues involved in the two sources of water are substantial and make an easy selection difficult. Surface water is usually soft water (makes good suds for washing and showering), while groundwater tends to be hard water and may not provide good suds for washing and showering without additional treatment, which would add a few more cents per thousand gallons to its cost. The iron-stained toilet bowl is an indication of the high iron content of the supply. Vigilant cleaning will eliminate any permanent staining. Water softening units, if not maintained properly, can cause increased sodium and chlorides in shallow groundwater systems where leachfields are constructed in a relatively thin soil interval overlying fractured sandstone or other rocks.

The Contamination Factors

Surface water is vulnerable to widespread contamination by accidents involving railroad chemical tank cars or trucks and intentional contamination by a disturbed person or radical group. It is also subject to bacterial contamination from septic tanks surrounding the reservoir. Some dams are subject to breaching by flooding. Although chances are relatively small, the impact of any such occurrences would be widespread and paralyzing to the local residents involved.

Groundwater is not as vulnerable to widespread, rapid contamination from surface spills as surface water, but groundwater is subject to subsurface contamination from oil and gas wells (both abandoned fields as well as operating fields, road-salting activities, malfunctioning individual water-softening systems, and from nearby gasoline stations). Since 1991, U.S. EPA requires that all large-scale drinking-water supplies (surface water as well as groundwater) are to be tested on a regular basis for a number of potential contaminants, such as benzene, and for pesticides, and other chemical constituents. Individual water-supply systems have no such regulations.

The design and construction of water-supply wells and groundwater monitoring wells have been well developed in the technical literature of the field for many years to minimize contamination as a result of faulty well construction. The safety of drinking water from individual wells in the rural setting is the responsibility of the home owner. However, cases of bacterial contamination (by E. coli, etc.) of such wells are known and indicate that young children and older persons are at serious risk. Regular testing and simple treatment should be a part of every water-supply system in rural areas, especially where septic systems, cattle and other agricultural factors may be present. Water-softening systems also need to be operated properly, as indicated above.

Groundwater quality will vary from region to region because of differences in the local geology of the aquifers produced as a water supply. Taste and odors may, from time to time, become a problem in smaller groundwater supplies where operators are not present on a continuous basis to monitor water-treatment systems, large and small. Slight changes in regulation of the chlorinating equipment can affect the taste (and odor) of the produced water.

Many wells also develop nonpathogenic iron and manganese bacteria that can affect the water by creating taste and odor anomalies. Sulfate-reducing bacteria, for example, can develop in a supply well, which impart additional taste and odor problems. The familiar “rotten eggs” odor arises when hydrogen sulfide is produced in very small quantities from the sulfate-reducing bacteria living in the anaerobic microcosms under crusts of aerobic iron-oxidizing bacteria. Both problems will tend to give the water a yellow-brown to light orange appearance. Regular monitoring of the water’s inorganic chemistry, combined with appropriate water treatment, can control such problems. The cost will depend upon their severity (add an additional $0.02 to $0.10 per thousand gallons of raw water produced).

Water-well maintenance is a critical factor in controlling water quality. Many individual rural water wells are not maintained appropriately or on a regular basis. In addition, many wells are initially located down-gradient from the septic tank and leachfields. Well maintenance usually consists of regular checkups of the downhole conditions of the well screen or intake, the submersible pump and motor, wiring, and fittings, For all practical purposes, the well should be maintained in a similar manner to that of the family car. Too often, however, the well is forgotten until a problem in quantity or quality develops. In municipal utility wells, maintenance is usually performed on a regular basis by the operator/contractor. With the EPA monitoring requirements in place, water quality can been monitored effectively. Before 1991, monitoring small water supplies were difficult for states to conduct on a regular basis without an indication of a problem with water quality.

On the other hand, some common types of contamination released from leaking underground storage tanks of service stations and other sources have been found to naturally degrade with time with the aid of specialized bacteria that are indigenous in the contaminated aquifer. The down-hole, well-screen photograph on the right above illustrates a properly-maintained well screen. The photograph to the left shows a well in need of maintenance. The reddish brown color is caused by iron bacteria and, under the iron scale, sulfate-reducing bacteria typically exist in low concentrations. As indicated previously, the former is often responsible for staining of porcelain fixtures and poor taste, while the latter if often responsible for the rotten-egg odor of the water. Both problems can be managed but if not controlled, a new well may be required to solve the problem.

Other issues can be involved that require professional input on: tank corrosion, water quality (water chemistry and microbiology), toxicology, and industrial health. I2M Associates can provide technical support in these areas.

See Mr. Campbell’s white paper of the late 1990s titled The Role of Environmental Technology in Developing, Maintaining, and Protecting Ground-Water Supplies in the 21st Century.

Dewatering for Construction and Mining

Dewatering is usually required to control subsurface water when excavating for construction purposes (i.e., for buildings, pipelines, etc.). Dewatering is also required in surface or underground mining of coal, lignite or other natural resources. Both methods must control the near-surface water in order to remove the commodity for processing into a product required by society as with open-pit mining.

To review the technical literature on these technologies and techniques, search “water supply” and related key words in the I2M Web Portal (here).

Feel free to contact us to discuss your project needs or to arrange a speaking engagement by one of our Associates for a professional training session, a technical conference, society meeting, or for a graduation ceremony or other function where the knowledge and experience of our Associates may be of interest to your group.

The I2M Principals responsible for this discipline’s activities are:

Michael D. Campbell, P.G., P.H.

Richard C. Bost, P.E., P.H.

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