Grappling with Groundwater
- By Jim Chaffee
- Apr 18, 2008
Do you know where your water supply comes from? Does your local government authority send it to your house from a nearby reservoir? Perhaps your district is served by wells?
[Click here for a map of the U.S. that illustrates the percentage of population served by groundwater.]
If it is, you are in good company. U.S. Environmental Protection Agency data indicates that more than 90 percent of the 158,000 public water systems use groundwater, not surface water. Private systems also tap into groundwater supply and a great number of Florida, Mississippi, and New Mexico residents drink groundwater.
This water supply is appealing because it
• is cost effective.
• has been less susceptible to contamination and excursions than surface water.
• typically requires minimal treatment.
Even today, many communities can hire a well driller, drill the well, confirm there is no bacterial contamination, install a pump, and start supplying customers. Capital costs may range from 30 cents to 50 cents per gallon for these types of projects and have minimal operating costs.
Most systems provide basic disinfection, although there are some groundwater supplies that are not disinfected even today. More extensive treatment typically involves aesthetic rather than health-related issues commonly associated with dissolved inorganic compounds, such as iron, manganese, hydrogen sulfite, salinity, and hardness.
The city of Madison, Wis., has a high-quality groundwater source. The city's water utility serves a population of 220,000 and operates a system supplied by 23 groundwater wells with capacities of up to 3 million gallons per day (mgd) each. In 2006, after it investigated treatment alternatives to solve a discoloration problem caused by elevated concentrations of manganese in one of its newest wells, the city selected a treatment approach using a specially designed high-rate filter to remove the contaminant. With an estimated capital cost of $2.1 million and a supply capacity of 3 mgd, the cost to treat this aesthetic issue was approximately 70 cents a gallon.
As with most good things, groundwater's use and management is changing, and public water suppliers are finding new challenges to overcome. In recent years, suppliers have been grappling with health-related water quality issues and more stringent regulations as well as dwindling supplies and competing uses (agriculture use and resource protection).
The expanded water quality monitoring requirements that took effect in the late 1970s eventually identified a number of manmade as well as naturally occurring contaminants in groundwater with potential health risks. In response, the regulations of the Safe Drinking Water Act established new and more stringent maximum contaminant levels for synthetic and volatile organic chemicals, nitrates, agricultural chemicals, as well as naturally occurring substances such as radionuclides and arsenic.
As EPA increased its enforcement of public drinking water quality standards, regulations intended for surface water suppliers actually affected groundwater suppliers. Surface water treatment rules requiring filtration as a microbial barrier, for example, also applied to groundwater systems "under the influence" of surface water. These systems are commonly referred to as "groundwater under the influence" (GWUI). Many groundwater systems are located in shallow alluvial aquifers close to and interconnected with surface waters, and as a result, can fall under the GWUI classification. If a shallow well is influenced by surface water, it can also contain elevated concentrations of dissolved organics that cause potentially harmful byproducts of disinfection to be formed in the treated water. To address this, EPA developed disinfection byproducts standards primarily intended for surface water suppliers.
The most recent EPA regulation impacting groundwater suppliers -- appropriately titled the "groundwater rule" (GWR) -- was published in November 2006 and requires compliance beginning in 2009. This rule is intended to protect consumers from bacteria and viruses that may occur in groundwater as a result of fecal contamination by requiring disinfection of "at risk" supplies. EPA estimated that the total initial capital cost to water suppliers of complying with this new regulation will be up to approximately $700 million.
The city of Mankato, Minn., manages its groundwater supply resources to address a broad range of issues. The city's supply principally comes from horizontal collector wells constructed in shallow alluvial deposits along the Minnesota and Blue Earth rivers and deep fractured bedrock aquifers. The shallow wells are classified as GWUI requiring filtration, and the groundwater there is susceptible to high nitrate concentrations and dissolved organics that can cause disinfection byproducts. The bedrock aquifer produces water with elevated radionuclide concentrations. The two sources are combined and treated in a central softening plant where various treated water goals are met by a combination of treatment, removal, and blending schemes. The city is implementing a $35-million capital improvement program for supply and treatment that includes expanding plant capacity and installing membrane filtration.
Where conventional groundwater supplies are scarce, water suppliers must become more innovative in exploring resource alternatives. Two apparent trends include tapping brackish water supplies and actively managing groundwater systems.
Performance improvements and lower costs have made desalination of brackish groundwater more economically feasible. Florida, Texas, California, and some coastal communities along the East Coast are trying this tactic. Along the coast, beached wells or estuaries provide a new untapped groundwater supply that often requires minimal pretreatment and can be effectively treated using medium-pressure membrane technology. For example, the Hilton Head (South Carolina) Public Service District is designing and building a 3-mgd desalination plant. Hilton Head turned to desalination as the 11 inland groundwater wells that supplied its water have been impacted by salt water intrusion.
The landscape for groundwater suppliers will continue to change, but groundwater will always be a vital source of public water supply. A broader range of regulatory challenges, water quality issues, increasing populations, and dwindling supplies mandates that communities take a more scientific, thoughtful, and creative approach to managing their groundwater resources.