Karst Terrain and Groundwater

 

The carbonate-rock aquifers are the predominate aquifer in the Valley and Ridge (V&R) of Virginia; however, there are areas in the Piedmont and Blue Ridge (P&BR) that also contain carbonate-rock aquifers. In total the carbonate rock aquifers underlie an area with a population of more than 40 million people in 10 states.

Where carbonate rocks are exposed at land surface or are overlain by only a thin layer of confining material they are easily dissolved by rain. As rain falls it absorbs some carbon dioxide from the atmosphere and from organic matter in soil. As the water percolates through the soil the weak carbonic acid water dissolves limestone and dolomite by enlarging pores between grains of limestone or fractures in the rock.

Over time these openings become larger as more of the acidic water moves through the aquifer; eventually the openings may be tens of feet in diameter. The end result of dissolution of carbonate rocks is a type of topography called karst- characterized by caves and sinkholes.

Water-supply wells drilled into the carbonate aquifers in karst terrain are generally more productive than wells that tap other rock types. The carbonate aquifers, due to the presence of dissolution channels, are very vulnerable to contamination from the surface. The carbonate aquifers are particularly vulnerable where sinkholes allow for the relatively rapid movement of contaminants into and through the aquifer. In some areas, the carbonate aquifers are locally isolated from the surface by thick layers of clay or shale that can impede the downward movement of water and contaminants.

The carbonate aquifers of the Appalachian Valley and Ridge Province, formed during Appalachian mountain building, have highly variable karst aquifer characteristics. The Valley and Ridge, Piedmont, and Blue Ridge Aquifers demonstrate karst features such as caves, sinkholes, sinking streams, and conduits. They are still used as a major drinking water supply for individuals and public supply, but without careful management these wells can become problematic.

The combined Valley and Ridge and Piedmont and Blue Ridge aquifers of all type rank second in the Nation as a source of groundwater for private domestic supply, providing about 470 million gallons per day (Arnold and others, 2016). The Valley and Ridge and Piedmont and Blue Ridge aquifers are also an important source of public supply, providing about 195 million gallons per day. Land use overlying the Valley and Ridge and Piedmont and Blue Ridge aquifers is mostly undeveloped (49 %), agricultural (35 %), and urban land (17 %).

Valley and Ridge and Piedmont and Blue Ridge aquifers in Virginia and were evaluated by the USGS National Water-Quality Assessment Project, which began in2012 and continued through 2021. Below are excerpts from that evaluation. The above information was taken from the USGS Groundwater Atlas of the United States.

Samples were analyzed for 34 trace elements and major and minor ions. Contaminants from this group were detected at high concentrations in about 10 % of the study area (at the depth zone used for public supply) and at moderate concentrations in about 5 %. Arsenic, manganese, and strontium were the only trace elements detected at high concentrations.

Samples were analyzed for eight radioactive contaminants, of which four have human-health benchmarks. Radioactive constituents were detected at high levels in about 3 % of the study area, but were not detected at moderate levels. Gross alpha activity was the only constituent detected at high concentrations.

Samples were analyzed for five nutrients, of which two have human-health benchmarks. Common sources of nutrients include fertilizer applied to crops and landscaping, seepage from septic systems, and human and animal waste. Nutrients were detected at high concentrations in about 2 % of the study area and at moderate concentrations in about 11 %. Nitrate was the only nutrient detected at high concentrations.

Some constituents affect the aesthetic properties of water, such as taste, color, and odor, or can create nuisance problems, such as staining and scaling. Samples were analyzed for 11 constituents that have SMCLs. One or more of these were present at high concentrations or values relative to the SMCL in about 15 % of the study area and at moderate concentrations in about 18 %.

Total dissolved solids (TDS) concentration is a measure of the salinity of the groundwater, and all water naturally contains TDS as a result of the weathering and dissolution of minerals in rocks and sediments. The TDS concentrations can be high because of natural factors or as a result of human activities, such as applications to the land surface of road salt, fertilizers, or other chemicals in urban or agricultural areas. The TDS concentrations were high in about 5 % of the study area.

Iron and manganese were both present at high concentrations relative to the SMCL in about 5 % of the study area. Sulfate was present at high concentrations in about 2 % of the study area. In a few samples, the pH of groundwater was not in the SMCL range of 6.5–8.5. In those cases, the pH was less than 6.5; such waters are considered acidic and potentially corrosive.

VOCs were detected at moderate concentrations in 2 percent of the study area. The only VOC detected at moderate concentrations was chloroform.

Samples were analyzed for 227 pesticide compounds (pesticides and their breakdown products), of which 119 have human-health benchmarks. Pesticide compounds were not detected at high or moderate concentrations in the study area.

Sinkholes

from Sinkholes, West-Central Florida USGS

Sinkholes can vary from small shallow depressions in the earth to holes that are hundreds of feet deep and cover hundreds of acres. Some sinkholes even hold water and form natural ponds and lakes. Typically, sinkholes form so slowly that little change is seen in one's life- time, but they can form suddenly when a collapse occurs. Such a collapse can have a dramatic and devastating effect if it occurs in an urban or suburban setting as recently happened in Hillsborough County near Tampa, Florida when a sinkhole opened beneath a house swallowing a man and his bedroom. The body was never recovered and the house was demolished. Western central Florida has a long history of sinkholes and because of geology and groundwater pumping is  particularly susceptible to sinkholes. In the water well fields for St. Petersburg located in Hillsborough County and surrounding counties sinkholes have occurred in conjunction with development of each of the well fields as well a throughout the region. Sinkhole formation is highest during dry months of the year and during drought, but overall appear to be increasing in frequency according to the U.S. Geological Survey, USGS, though there might be some reporting bias to the data.   

A landscape that forms sinkholes, sinking streams, caves, and springs is called a karst landscape. A karst landscape most commonly develops on limestone, but can develop on several other types of rocks, such as dolostone (magnesium carbonate or the mineral dolomite), gypsum, and salt, which are types of evaporates rocks. Rain is naturally mildly acidic, and slowly over time the weakly acids rainwater dissolves these deposits creating fissures. The deposits are highly permeable, and surface water passes through them quickly to underlying aquifers, eating away at the limestone and evaporates bedrock. Overtime this creates the voids that become sinkholes. There are three general types of sinkholes: dissolution sinkholes—depressions in the limestone surface caused by the erosion of limestone by rain; cover-subsidence sinkholes—formed as overburden materials gradually fill below surface fissures formed by the infiltration of rain; and cover collapse sinkholes—which occur in limestone terrain with a thick overburden or mantle after the fissures forms large cavities and the cover materials collapse into the subsurface voids. This third type of sinkhole is what occurred last week near Tampa.

Hundreds of collapse sinkholes of various sizes occur throughout the country each year and start unnoticed when infiltrating water or groundwater flowing in the subsurface creates a void where soil is washed away. Eventually the void or hole grows large enough that the soil above it can no longer bridge it. The soil bridge then suddenly collapses into the void below and a sinkhole forms. Often this happens when the water level that has been exerting an upward pressure- helping to hold up the soil bridge falls. This process usually takes many years to occur in nature, but it can be aggravated by human activities. Any activity that increases the amount of water flowing into the subsurface can speed up this process. Parking lots, streets, altered drainage from construction, irrigation, leaking swimming pools and roof guttering are some things that can increase runoff; even severe weather can cause sinkholes.

from the Florida USGS office

The most damage from sinkholes tends to occur in Florida, Texas, Alabama, Missouri, Kentucky, Tennessee, and Pennsylvania thought large areas of the United States are underlain by evaporates rocks (salt, gypsum, and anhydrite) and carbonates (limestone and dolomite), the rock types that are most susceptible to being dissolved away by water. Even when evaporite rocks are buried at great depths, in so called Mantled Karst terrain sinkholes can form. These sinkholes are the most sudden when the mantle give way. The western central portion of Florida is an area of Mantled Karst terrain, but most of Florida is prone to sinkhole formation because it is underlain by thick carbonate deposits and is so rich in groundwater. Development and overuse of the groundwater resources for municipal, industrial and agricultural water supplies has resulted in falling groundwater levels that play a role in sinkhole formation as well as development.

According to Ann B. Tihansky of the U.S.G.S. in Tampa and author of Sinkholes, West-Central Florida,“Induced sinkholes are generally cover-collapse type sinkholes and tend to occur abruptly. They have been forming at increasing rates during the past several decades and pose potential hazards in developed and developing areas of west-central Florida. The increasing incidence of induced sinkholes is expected to continue as our demand for groundwater and land resources increases. Regional declines of ground-water levels increase sinkhole occurrence in sinkhole-prone regions.” The sinkhole prone regions of the country can be seen below.All of Florida is karst terraine, but as can be seen below karst terrain also covers much of the Valley and Ridge Province of Virginia in the western third of the state. Small karst areas occur in the Cumberland Plateau, Piedmont and even the Coastal Plain provinces.

From USGS

If you have questions or worries about sinkholes, settling or earth movement in your yard, Florida has an excellent question and answer web site. 

Raspberry Falls Bearing the Cost of Water Problems in Karst Terrain

The karst area of Loudoun County is contained within the limestone overlay district that was created in 2010 when Loudoun County Board of Supervisors approved the re-adoption and re-enactment of the Limestone Overlay District (LOD). This district is the area of the county generally north of Leesburg and east of the Catoctin Mountains which is underlain by limestone conglomerate bedrock and runs north along Route 15. The LOD is really an amendment to the zoning to try and address the ecological and environmental challenges associated with karst terrain. The LOD attempts to ensure that the groundwater supply is capable of supporting needs of the eventual inhabitants of new subdivisions and the land can support the septic needs of current and future residents without impacting the water supplies of existing residents and creating sinkholes that could endanger their properties. Karst terrain is fragile and ignoring the limits of natural systems can have serious consequences.

Raspberry Falls is a clustered development around a golf course in the LOD of Loudoun County. While clustered development usually involves fewer disturbances to natural landscape, and is encouraged in low impact development designs, golf courses are not a low impact design feature. The groundwater pumping to feed the homes and water the golf course may create or exacerbate problems in karst terrain, especially during droughts. The turf management herbicides can also be a problem. The Raspberry Falls development was originally approved for 206 homes and currently has, I believe, 134 homes that are served by a community water system consisting of two wells built out by the developer and operated by Loudoun Water. The system has been plagued by the bacterial problems that are a common problem with karst terrain. Fractures in the overlying limestone become enlarged over time and provide a direct route of surface water to groundwater. Sinkholes proved another direct path for surface contaminants to enter the groundwater as do sinking streams and rivers. The faster water moves into the ground the higher the likelihood that bacteria will remain alive and nitrate is not going to denitrify. When that happens the groundwater is deemed to be, Groundwater Under is the Direct Influence of Surface Water or GUDI. A water source is determined to be GUDI if more than 10% of total coliform numbers exceed 100 cfu/100 mL.

One of the water supply wells for Raspberry Falls was determined to be GUDI by the Department of Health and taken out of service and replaced this past year by a new well at a cost of almost a million dollars. Replacing the well solved the problem for the short term, but experience in the western third of Virginia has demonstrated that the GUDI condition could impact the other wells. Local septic systems did not appear to be the source of bacterial contamination, but E coli numbers were not broken out in the raw water data by Loudoun Water, only total coliform. However, wastewater in Raspberry Falls is collected and treated to a at the community wastewater treatment plant (WWTP). Most organic material and nutrients are removed through biological treatment before being disinfected and discharged to an unnamed tributary of Limestone Branch.

Though the GUDI well was replaced with a new well (and right now the water meets all the standards of the Safe Drinking Water Act), additional treatment beyond the current chlorine disinfection of the drinking water is being considered because of the relatively easy connection of surface water to groundwater, the unconfined nature of the groundwater aquifer and probably the immediate problems with turbidity that the new well experienced. Two options were considered and studied by Hazen and Sawyer (Loudoun Water’s consultant): the extension of municipal water from Leesburg into the Rural Policy Area that encompasses Raspberry Falls or the installation of membrane filtration for all Raspberry Falls water supply wells as an additional water treatment step. The extension of the municipal water pipeline would involve an amendment to the Loudoun County Revised General Plan to authorize extending the pipeline and the Town of Leesburg would need to accept ownership and operation of the Raspberry Falls Community Water System.

The costs associated with the installation of membrane filtration and operation were $4 million for purchase and installation and $67,000 additional in annual operation costs for Raspberry Falls only. Loudoun Water estimates that the cost per lot would be an additional $1,830/year for membrane filtration. The Town of Leesburg estimated the construction cost of the pipeline at $7.5-$8 million, with annual operating costs of $418,000 and an annual cost per lot of $4,260/year. The recent water rate increase does not take into consideration either of the options under consideration. The membrane filtration was the cheaper option, and would still carry significant costs that would ultimately be paid for by the water system customers. Loudoun Water appears to prefer the cheaper membrane filtration system, and though residents might suspect business concerns took precedence over water quality issues, it may be an excellent solution given what is known about the source water quality. Loudoun Water has requested that Loudoun County and the Town of Leesburg decide whether to pursue the pipeline extension no later than May 2012 otherwise they will proceed with installation of membrane filtration at Raspberry Falls.

There appears to be many residents who strongly support the pipeline/ Leesburg water solution despite the higher cost. Appropriately, many residents are only concerned about obtaining the best water supply possible, but the pipeline may not be that answer. Personally, after reviewing the US Geological Service (USGS) raw water studies and the USGS and US Fish and Wildlife (USFW) studies into skin lesion on bass in the southern branch of the Potomac River, I would hesitate to pay extra to drink water sourced from the Potomac River without advanced nanomembrane filtration. The USGS found fish suffering from a variety of lesions. Some fish had bacterial lesions, some fungal lesions, and some fish had parasite. The USGS concluded that there was no specific cause of the lesions and that the fish appeared to be immunosuppressed so that any pathogen in the water could attack the fish. A series of studies were performed over a period of years. During the investigation it was discovered that male fish had immature female egg cells in their testes and the females had lowered levels of an essential protein in the formation of eggs. The bass suffering from lesions were intersexed.

It had previously been demonstrated that estrogen and estrogen mimicking compounds can cause intersex. The occurrence of intersex among the lesioned fish prompted further studies. The study found the problem of endocrine disruption in fish to be widespread in the limited study area of a portion of the Chesapeake Water Shed and Potomac River, but increased in proximity to and downstream of the waste water treatment plants. Chemical sampling that took place along with the fish sampling found higher concentrations of waste water chemicals near the waste water plants. Pesticides, herbicides and their breakdown products currently used in agriculture were detected at all locations. Hormones were not detected in the samples, but analysis using yeast screening assays found estrogenic endocrine-disrupting chemicals at all locations their specific source is not yet known. None of these chemicals are currently regulated under the Safe Drinking Water Act, SDWA, and so would not be tested for or treated by the Leesburg town water treatment system except by happy coincidence.

Before the residents of Raspberry Falls choose a solution to their water problems they should consider carefully the quality of the source water and finished water. USGS groundwater source studies have also found the presence of the gasoline additive MTBE, the solvent 1,1-dichloroethane, and the herbicide breakdown products from alachlor and atrazine in a significant percentage of groundwater supply wells in unconfined aquifers. The herbicide degradates are not regulated by the USEPA in drinking water under the SDWA, so these contaminants are not tested for in drinking water and there are no known health screening levels. Though these herbicides are widely used in agriculture and turf management for golf courses and the herbicide degradates may be regulated by USEPA under the Federal Insecticide, Fungicide, and Rodenticide Act (FIFRA). Though whatever solution Loudoun County Supervisors and the residents of Raspberry Falls choose will have additional costs associated with it, this is a rare opportunity to select your source water and potentially your treatment method after purchasing your home. Performing extensive source and finished water analysis before making a final decision might be very worthwhile in this instance.