|The possibility that chloride may also be derived from septic
drain fields implies that nitrate and other constituents of septic leachate should be too.
In fact, data from Pocatello wells (Welhan et al., 1996) indicates that chloride is
correlated with nitrate, sulfate, and calcium - all constituents of septic leachate.
A number of Pocatello wells display nitrate and chloride concentrations which have shown statistically significant increases over a 15-year period. This is troubling because even in these well-flushed portions of the aquifer system, nitrate is entering rapidly enough to allow concentrations to build up over time.
Elevated nitrate concentrations are generally found in the northern valley, although increasing trends exist in the central and southern valley, as well. A large amount of information on ground water nitrate is also available from a DEQ study conducted in the eastern tributary aquifer (Blackcliffs Trailer Park; DEQ, 1994). This area has the highest nitrates in the valley, with some wells far in excess of the EPA safe limit of 10 milligrams per liter, and leakage from it is suspected for the occurrence of these septic salts in the Ross Park wells and possibly nearby wells in the vicinity of the Highway Pond.
Because this tributary aquifer is a thousand times less permeable than the municipal aquifer, it is poorly flushed by natural ground water through-flow and septic-derived salts have built up to high levels over a disturbingly wide area. Whether this situation is peculiar to this one area or is symptomatic of conditions in other low-permeability tributary aquifers (such as the Johnny Creek area, the Gibson Jack bench, and the Mink Creek area), is presently not known.
However, to allow areas not yet so loaded with septic leachate to attain such levels through inadequate septic sewage management policy is clearly not in the interests of controlling or eventually reversing the existing nitrate trends in the main valley aquifer.