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.

The Piedmont and Blue Ridge Groundwater

About half of the nation’s population relies on groundwater for drinking water. As the nation’s population grows, the need for high-quality drinking-water supplies becomes ever more urgent. The USGS has identified 68 principal aquifers in the United States, these are regionally extensive aquifers that are used as sources of drinking water.

Groundwater pumped from these primary aquifers provides nearly 50% of the nation’s drinking water. Twenty of these principal aquifers account for about three quarters of the nation’s groundwater pumped for public supply. These aquifers also provide 85 % of the groundwater pumped for domestic (private) supply. Three of these principal aquifers are in Virginia and were evaluated by the USGS National Water-Quality Assessment Project, which began in2012 and continued through 2021. Below are excerpts from the evaluation of the Piedmont and Blue Ridge aquifers and information taken from the USGS Groundwater Atlas of the United States.

The Piedmont and Blue Ridge crystalline-rock aquifers underlie an area with a population of more than 25 million people in 11 states (map). The Piedmont and Blue Ridge crystalline-rock aquifers, together with the other rock types in the Piedmont and Blue Ridge regions, rank second in the Nation as a source of groundwater for private domestic supply, providing about 360 million gallons per day (Arnold and others, 2017a).

These aquifers are also an important source of public supply, and about 92 million gallons per day are pumped for that use. Land use overlying the Piedmont and Blue Ridge crystalline-rock aquifers is mostly undeveloped (71 %) and agricultural (19 %). The cities of Atlanta, Georgia, and Charlotte, North Carolina, overlie the aquifers, as well as suburbs of Richmond, Virginia; Washington, D.C.; Baltimore, Maryland; and Philadelphia, Pennsylvania.

The Piedmont and Blue Ridge Provinces are underlain by three principal types of bedrock aquifers. In order of decreasing area, these are crystalline-rock and undifferentiated sedimentary-rock aquifers, aquifers in early Mesozoic basins, and carbonate-rock aquifers. Unconsolidated aquifers that are part of the surficial aquifer system overlie the bedrock aquifers locally in Pennsylvania and northern New Jersey.

Crystalline-Rock and Undifferentiated Sedimentary-Rock Aquifers are the most widespread aquifers in the Piedmont and Blue Ridge Provinces. These aquifers extend over about 49,000 square miles, or about 86 % of the area, of these provinces. Most of the rocks that make up crystalline-rock and undifferentiated sedimentary-rock aquifers are crystalline metamorphic and igneous rocks of many types. The main types of crystalline rocks are coarse-grained gneisses and schists of various mineral composition; however, fine-grained rocks, such as phyllite and metamorphosed volcanic rocks, are common in places.

Unconsolidated material called regolith overlies the crystalline-rock and undifferentiated sedimentary-rock aquifers almost everywhere. Because the regolith material varies greatly in thickness, composition, and grain size, its hydraulic properties also vary greatly. However, the regolith is more permeable than the underlying bedrock. Water in the bedrock is stored in and moves through fractures, which form the only effective porosity in the bedrock.

Early Mesozoic rift basins are spread out in the Piedmont Province and occupy about 9 % of the combined area of the Blue Ridge and the Piedmont Provinces. Aquifers in early Mesozoic basins are primarily in three major basins-the Newark Basin in New Jersey and Pennsylvania is the largest basin and the one from which the most ground water is withdrawn; second largest is the Gettysburg Basin of Pennsylvania and Maryland; and third is the Culpeper Basin of Virginia.

The Culpeper Basin of northern Virginia and Maryland is an elongate, fault-bounded trough that trends north-northeast from the southern border of Madison County, Va., about 90 miles to Frederick County, Md. All the formations in the basin are part of the Culpeper Group. The lower part of the group consists of sandstone, siltstone, and conglomerate of Late Triassic age; the upper part consists of Lower Jurassic sedimentary rocks and interbedded basaltic lava flows.

The water in the Culpeper Basin is the least impacted by iron, manganese and sulfate in the region and of only moderate hardness. My home overlies a section of the Culpeper basin that runs through all but one small corner of northwestern Prince William County. I chose this area for the water. It requires no treatment.

Carbonate-Rock supports the largest aquifers in the Piedmont and Blue Ridge. Limestone, dolomite, and marble of Paleozoic and Precambrian age form carbonate-rock aquifers that extend over about 3 % of the Piedmont and the Blue Ridge Provinces. Although these carbonate rocks are of small extent, they are significant local sources of water. Carbonate-rock aquifers are in five areas of the Piedmont and the Blue Ridge Provinces. In addition to these areas, small, isolated elongate stringers of limestone and marble form minor aquifers locally, particularly in Virginia, and generally trend parallel to the Blue Ridge front.

Recharge is highly variable in the Blue Ridge and the Piedmont Provinces because it is determined by local precipitation and runoff, which are highly variable and are influenced by topographic relief, ground cover, compaction and the capacity of the land surface to accept infiltrating water. 

Most of the Piedmont and the Blue Ridge Provinces are covered by regolith. Compared to the Blue Ridge, the gentler topographic relief of the Piedmont and less precipitation make the Piedmont less subject to rapid denudation than the Blue Ridge and thus favor the accumulation of a thicker regolith. The combination of large areas of thin regolith and dense bedrock with minimal permeability in the Blue Ridge Province do not favor large amounts of ground-water recharge. These areas have a limited ability to provide water.

Almost all recharge is from precipitation that enters the aquifers through the porous regolith. Much of the recharge water moves laterally through the regolith and discharges to a nearby stream or depression during or shortly after a storm or precipitation event. Some of the water, however, moves downward through the regolith until it reaches the bedrock where it enters fractures in crystalline rocks and sandstones or solution openings in carbonate rocks.

The USGS Aquifer Studies were designed to evaluate groundwater used for public supply prior to any treatment. Groundwater quality was assessed by comparing contaminant concentrations to regulatory limits established for drinking water quality.  Trace elements and major and minor ions are naturally present in the minerals of rocks, soils and sediments, and in the water that comes into contact with those materials.

The USGS sampled 60 wells at depths that a used for public supply wells: 150-700 feet beneath grade. Samples were analyzed for 90 VOCs, of which 38 have human-health benchmarks. VOCs were detected at moderate concentrations in 5 percent of the study area but were not detected at high concentrations. Compounds detected at moderate concentrations were the disinfection byproduct chloroform and the solvent trichloroethylene (TCE).

Manganese was found to be present at high concentrations relative to the SMCL in about 15 % of the study wells. Iron was present at high concentrations relative to the SMCL in about 12 % of the wells.

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

In some areas, the pH of the groundwater was not in the SMCL range of 6.5 to 8.5. The pH did not meet the standard in 35 % of the study area, typically because it was less than 6.5, which is acidic and potentially corrosive.

The total dissolved solids (TDS) concentration is a usually considered a measure of the salinity of the groundwater, though all water naturally contains TDS as a result of the weathering and dissolution of minerals in rocks and sediments. Concentrations of TDS can be high because of natural factors or as a result of human activities such as applications of road salt, fertilizers, or other chemicals to the land surface in urban or agricultural areas. Concentrations of TDS were high in about 3 % of the study area. Chloride, fluoride, and sulfate—constituents that also contribute to TDS concentrations—were detected at moderate, but elevated concentrations.

Radioactivity is the release of energy or energetic particles during spontaneous decay of unstable atoms. Most of the radioactivity in groundwater comes from the decay of isotopes of uranium and thorium that are naturally present in minerals in aquifer materials. Samples were analyzed for eight radioactive constituents, of which four have human-health limits for drinking water. The USGS found radioactive constituents were present at high levels in about 30 % of the study area and at moderate levels in about 17 %. Radon (using the proposed alternative maximum contaminant level of 4,000 picocuries per liter) and gross-alpha activity were the only constituents that were present at high concentrations. Radium (combined concentration of Ra-226 and Ra-228 isotopes) was detected at moderate concentrations in 2% of the study area.

Nutrients are naturally present at low concentrations in groundwater; high and moderate concentrations (relative to human-health benchmarks) generally result from human activities. Samples were analyzed for five nutrients, of which two have human health benchmarks. Common sources of nutrients, aside from soils, include fertilizer applied to crops and landscaping, seepage from septic systems, and human and animal waste. No nutrients were detected at high concentrations in the study area. Nitrate was detected at moderate concentrations in about 3% of the study area.