Regional Aquifer Systems: Examples
Regional Aquifer Systems: Examples
Ground water flow systems extend over a wide range of scales, from small perched aquifers that may supply water for a single-family, to regional rock formations that span thousands of km and cross several states (Figure 18). These regional systems supply water for irrigation and domestic uses in many areas, especially in semi-arid and arid parts of the American West and coastal population centers along the East coast (remember Module 1, figures 10-12?). These regional systems commonly consist of several layered sedimentary formations and may extend to several kilometers in depth. The U.S. Geological Survey has compiled detailed studies of regional aquifer systems across the U.S., with useful information about climate, recharge, subsurface geology, use, and problems related to water quality or quantity (a list and links for each of the principal regional aquifers in the U.S can be found at USGS Groundwater Information [1]. A detailed atlas with information about the major aquifer systems in particular regions of the U.S. can be viewed at USGS Ground Water Atlas of the United States [2]. In this module, we will focus on a few example regional aquifer systems of particular relevance to the Northeastern and mid-Atlantic U.S. and the Central Valley of CA.
Valley and Ridge Aquifer System
Valley and Ridge Aquifer System
The Valley and Ridge aquifer system extends SW-NE across Central PA, West VA, and VA (Figure 19, purple area), and is the main water supply for much of this region. It is composed of layered Paleozoic sedimentary strata (shales, sandstones, and limestones) that were folded and deformed by a series tectonic collisions over 200 million years ago. The modern valleys in the Valley and Ridge province have formed where limestone, which is most susceptible to erosion, was exposed in the core of anticlines, or upfolds (Figure 20). The more resistant sandstones and shales form the regional ridges, like Mt. Nittany and Bald Eagle Ridge.
The principal aquifer unit in this system is the fractured limestone that underlies the valleys. As noted above, because it is fractured, it recharges rapidly, has a high fracture permeability, and wells drilled along the fractures are highly productive (c.f. Figure 17). Recharge is focused on the flanks of the ridges, where runoff flows over the less permeable shale and sandstone units and enters the groundwater through fractures or sinkholes above the limestone at the valley edges. Groundwater flow is generally toward the center of the valleys, and springs commonly feed the surface water systems. The water is characterized by a high hardness (Mg and Ca content; we’ll cover this in more detail in Module 7), derived from limestone dissolution. Dissolution of the limestone has formed extensive karst features (caves, caverns, sinkholes) throughout the region.
Atlantic Coastal Plain Aquifer System
Atlantic Coastal Plain Aquifer System
The Atlantic Coastal Plain aquifer system extends North-South along much of the Eastern portions of New Jersey, Delaware, Pennsylvania, Virginia, and North Carolina (Figure 19). It consists of a sequence of layered sedimentary aquifers (sands and gravels) separated by series of aquitards, all deposited starting around 100 million years ago and continuing today. The layers slope, or dip, to the East and extend offshore for tens of km beneath the continental shelf (Figure 21).
Recharge occurs by both natural and managed infiltration on land across much of the coastal plain; groundwater flow in the subsurface is mainly to the East along the sediment layers. One interesting consequence of this flow pattern is that there may be a sizable freshwater resource offshore that could be accessed by drilling in relatively shallow water on the continental shelf. During the last ice age, when conditions were substantially wetter than today and a nearly mile-thick ice sheet covered the northern extent of the aquifer system, recharge was probably even larger - and thus may have forced fresh water several tens of km offshore, where that “fossil” water may remain today!
The Atlantic Coastal Plain system is an important water source for domestic/municipal supply and industry in population centers throughout coastal North Carolina, Maryland, Virginia, Delaware, and New Jersey. However, concentrated, localized pumping has led to a reversal of flow direction (toward the wells instead of Eastward) in some of the aquifer units throughout the region. In addition to overarching concerns about the sustainability of withdrawals that exceed recharge rates, the flow reversal has led to local salt-water intrusion, whereby saline ocean water infiltrates the aquifer and in some cases renders it non-potable.
Central Valley/San Joaquin Valley Aquifer System
Central Valley/San Joaquin Valley Aquifer System
The Central Valley Aquifer system of Central CA lies in a large structural basin running approximately North-South, between the Coast Ranges to the West and the Sierra Nevada mountains to the East (Figure 22). The deep elongate basin is infilled with marine and continental sediments, primarily composed of interlayered sands and clays. The basin itself is formed by tectonic processes caused by East-West extension (these are the same forces that are causing continued uplift of the major mountain chains throughout the Basin and Range province of the Southwestern US, and which are one major control on orographic precipitation patterns in that region).
The continental deposits (Figure 22, orange) comprise the main aquifer units and range from one-half to over two miles in thickness. As is the case in the Valley and Ridge, the recharge is primarily focused around the valley perimeter as runoff over the flanks of surrounding high topography infiltrates and enters the groundwater system. Groundwater flow is primarily inward, toward valley center, with a component of flow down-valley to the North, parallel to surface water flow in the San Joaquin River.
The thick sedimentary sequence has formed a vast expanse of flat topography on the natural floodplain of the San Joaquin River. This, in combination with a mild climate that allows a year-round growing season, has made the Central Valley one of the most productive and largest agricultural centers in the world. The Central Valley aquifer system is highly utilized, primarily to augment limited allocations of surface water for irrigation. Since the mid-1920s, groundwater withdrawals have generally outpaced natural recharge to the aquifer, leading to dropping water levels, irreversible aquifer compaction, and land subsidence (as will be discussed in more detail next week, in Module 6.2). Until recently, groundwater withdrawals were neither heavily monitored nor regulated. However, in the face of an ongoing multi-year drought, a 2014 bill was signed into law that restricts pumping and implements groundwater sustainability plans (See What to Know about California's New Groundwater Law [4]; see also New California Groundwater Pumping Rules Signed Into Law [5]). Shallow aquifer units in the valley are also plagued by a wide range of water quality concerns associated with irrigation and return flow of irrigation water to the aquifer via infiltration; these include leaching of selenium, boron, and other constituents from soils; salinization; and high concentrations of pesticides and fertilizers. We’ll discuss all of these issues in more detail in upcoming modules about water quality and the effects of climate change.