Clearly, the deep artesian aquifer presents more potential for health problems than does the shallow aquifer. But the deep artesian aquifer is also a “better” aquifer because it supplies water more readily than does the shallow aquifer. As a generalization, the TDS content of the water increases with increasing depth. There is no point in drilling much deeper because the water becomes too salty to drink. Sodium (fluoride, and just about all other dissolved species, especially chloride) increases in concentration toward the southeast as is shown on the accompanying map. The reason for this fact is just bad luck. Approximately 35 million years ago an object from outer space hit the ocean around Cape Charles. The impact pulverized the rocks, making it easier for water to flow through them, and seawater rushed in to fill the crater. Therefore salty water is not only closer to the Northern Neck than it would otherwise have been had the impact not occurred, but it is easier for salty water to move to the northwest as we continue to lower the pressure in our artesian aquifers by withdrawal of water. There is no evidence from Health Department records that the process of saline intrusion has begun anywhere in either Northumberland or Lancaster counties yet.
The water in both our artesian aquifers is classified by geochemists as a “sodium-bicarbonate” type, which is common throughout the world.
Bicarbonate (the HCO3- ion) is also commonly expressed as “alkalinity.” The water in our artesian aquifers is thousands or tens of thousands of years old. With time, as rain moved down the eastward-sloping aquifers from near the fall line, or vertically across the confining layers, chemical reactions progressed. Acid-producing reactions, including the decomposition of organic material, caused calcium carbonate (shell) to dissolve, producing a calcium-bicarbonate type of water. Neutralization of the acid by calcium carbonate resulted in a “hard” alkaline water with a pH of about 8.5. Uptake of calcium and release of sodium by clay minerals (ion exchange) accounts for the “soft” sodium-bicarbonate composition of the water today. In contrast to artesian water, rain is acid, with a pH typically around 4. Most of our plants are adapted to our acid soils, and the use of sodium-rich alkaline deep artesian water for irrigation can cause problems with plant growth.
Various kinds of water treatment processes are available to remove the sodium and other dissolved substances from the water, or to replace them with other substances. All kinds of water treatment are expensive and maintenance-intensive, and are unnecessary on a whole-house basis. Drinking bottled water may be the least expensive alternative for people worried about health effects of water from our deep aquifers, and rainwater harvesting is a simple and inexpensive solution to small-scale irrigation problems. Eventually, perhaps a century from now, water quality will have degraded and water levels in wells will have dropped to the point that alternative sources of water, like reservoirs, will be necessary.
If you missed a NAPS Stewardship tip, they can all be found, along with links to other sources of information, at the NAPS web site. Previous steward ship tips have addressed shallow aquifers, deep aquifers and rainwater harvesting, as well as water withdrawal by Maryland causing wells along the Potomac to go dry.