Water Works 46 - 3/12/04

Aquifer Testing for Idiots

There’s a touch of poetry in the best technical literature on any subject. Newcomers usually miss that because we’re too busy stumbling over unfamiliar terminology while we try to catch the main points. Mastering both a new language and a new way of looking at some part of our world at once is hard enough. Expecting anyone to listen for the music of the spheres on top of that is asking a lot. The vocabulary of hydrogeology can be especially intimidating, but the branch of that subject called “stream morphology” is in a class of its own. Reach that chapter in any text, and suddenly all the key words seem to explain themselves.

Flashy creeks like the Lockatong meander. Where they bend, they carve up their outer banks when they flood, always reaching for sharper turns. Rubble torn from slumped, upstream banks builds point bars downstream. New channels braid themselves around the bars and around the debris every storm leaves in its wake. Sheeting flows and shooting flows in the current find new paths of least resistance at every new obstacle. Long-established channels throw up their own levees and impoundments. Bends that grow too sharp for their own good are short-cut into oxbows. Sometimes in a wasted bank you can spot the imbricated footings of an old point bar, looking like a buried stone wall toppled over, eye-level high in the creek, with the roots of a forty-foot tree in its grip where the creek ran when your grandfather was a boy.

Those of you with no patience for geologic time can watch all that happen after any soaking rain, without straying too far from a county road or even from your car. But from any vantage point along the Lockatong and the Wickecheoke, the most striking sight of all is how little water you see in those creeks when they’re not flooding.

Our creeks look parched when they’re not flashing partly because surface runoff from precipitation, also called “stormwater runoff,” makes up an exceptionally large portion of our local “hydrologic cycle.” That cycle has two other components, “aquifer recharge” and “evapotranspiration.” Aquifer recharge refers to the relatively small amount of water that migrates into the bedrock below our soils each year. That is what we drink. Evapotranspiration prevents about half of our annual precipitation from getting any farther than the soil. That is what plants use to grow in season. At all times of the year, the rest of the rain or snow that falls here flows into our creeks as stormwater runoff, often just called “runoff.” (32)

After runoff floods a stream, water continues to work its way there laterally through the soils for about a week. (37) From then until the next flooding, the water in the stream comes entirely from its bedrock foundation. The water-bearing rock of our local aquifers extends a uniform 300 feet or so below ground level from any surface elevation. (29, 31) The hydraulic force exerted by groundwater stored at the higher elevations of a watershed’s bedrock constantly pushes water stored at lower elevations down into its creek beds. (30) The water you see flowing in Lockatong Creek after two dry weeks is the visible tip of all the water stored in the Lockatong watershed’s aquifer, and it’s called “baseflow.”

The amounts of runoff and baseflow in a stream’s total flow are calculated through a process called “baseflow separation.” Reference works on the hydrologic cycle tell us that total stream baseflow must always equal total aquifer recharge. Once you compute a stream’s annual baseflow, you know how much recharge its watershed received that year. Dividing annual baseflow by the land area of the watershed gives the watershed’s annual recharge rate. For aquifers recharged by rain, like the Lockatong’s, recharge rates are normally expressed in inches of precipitation per acre of land per year. (2)

Now, remember those low-flow numbers I cited last week? (45) They tell us that Lockatong Creek suffered from chronic low baseflows more than 25 years ago, and that recharge rates for the Lockatong aquifer were equally low long before the NJDEP began mining the water we drink. Despite the obvious visual evidence of low flows and flashiness that have always characterized Lockatong Creek, the NJDEP never used Lockatong baseflows to set a safe limit for the water mining it has endorsed here for years.

There are only two ways to pump groundwater from an aquifer within safe limits. One is to calculate the aquifer’s recharge rate using stream baseflow and pump only as much water as the result allows. The other is to pump water until signs of stress appear – like shrinking baseflow in streams – and then cut back the pumping until things return to normal. But here the NJDEP chose option three: pump away, ignore evidence of an over-stressed water supply, and then pump more – and let the devil take the hindmost.

Ron Gutkowski