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.