TOXIC CHEMICALS AND DRINKING WATER

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Water in streams takes about two weeks (on the average) to make its journey from the headwaters to the ocean, while lakes hold a portion of their rainfall somewhat longer ... but ground water may be in place for thousands of years, and generally moves at an annual pace of less than ten feet. For that reason, contaminants can remain present at high concentrations long after entering ground water.

Yet another important characteristic of ground water is that this liquid is actually contained within rock and/or sediments, and therefore receives considerable filtration. (The image of underground rivers is accurate only in the loosest sense ... cavities filled with water actually are quite rare.) This filtration effectively eliminates biological hazards, but it's far less successful at stopping toxic chemicals. In fact, some particularly volatile synthetic organic substances (trichloroethylene is one of these) may move even more rapidly through ground water than does the H20 itself. There is also some concern that layers of subterranean sediment which are, for all practical purposes, impermeable to such substances as oil and salt may be far more permeable to this new breed of pollutant. Thus clay layers that had previously been considered adequate for "insulating" drinking-water aquifers from those used to house disposed-of waste (yes, toxic substances of all sorts are actually pumped underground in efforts to "get rid" of them) may already be proving inadequate to confine the volatile organic chemicals.

If you hope to become involved in analyzing the effects of a specific source of contamination, you must understand your local hydrology. You'll need to know, for example, whether surface water-which might contain pollutantsis passing into the ground water (a process known as aquifer recharge), running off to streams that may be recharge sources, or simply contaminating adjoining surface water. The relationship of water movement to your drinking source will be crucial to your ability to determine whether harmful chemicals are contaminating the water coming out of your tap.

WHAT SHOULD MAKE YOU NERVOUS?

PRIVATE WASTE DISPOSAL SYSTEMS: Septic tanks with drainage fields can release pollutants that may reach your drinking supply. A well should never be located closer than 100 feet to a leach field, and 200 feet would be much safer. It's also commonly recommended that the waste field be lower than the well site. Though this rule does have some merit, it certainly provides no guarantee of safety ... since water tables often rise to match geography, and pickup points for deep wells are, of course, far below the surface. An especially serious threat can be posed by a septic tank and drainage field that has been cleaned with trichloroethylene (the toxic carcinogen mentioned early in this article). TCE will do a fine job of removing grease from the concrete chamber, but if it isn't all removed, the volatile chemical will move quickly through the soil and into the water table.

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