My Aquifer > Groundwater in the LPRV > Is our aquifer contaminated?

 

Perhaps the most well-known example of ground water contamination in the lower Portneuf Valley is the trichloroethylene (TCE) plume (Figure 13) in the southern valley and in the vicinity of the old County landfill. A similar chlorinated solvent, perchloroethylene (PCE), is the cause of Chubbuck's water quality problems. Both originate from disposal on the land surface, and find their way to the water table where they are then carried by the movement of ground water.

Due to the rapid rate of ground water flow in the southern valley aquifer (up to 50 feet per day), the TCE plume at one time was advancing and affecting water quality in downstream wells at the rate of a mile per year. Several wells have been closed because their concentrations of TCE exceed safe limits; some remain closed, others have reopened. The plume still extends the length of the southern valley.

It is for these reasons that city water managers contracted with consulting engineers to draw up plans for special wells to capture the moving TCE, remove it from the water, and return clean water to the aquifer (this approach has been successfully adopted by the City of Chubbuck for one of its most contaminated wells). This plan has the advantages of curtailing the spread of TCE, protecting downstream wells from future contamination, and pumping part of the cleaned water into the water distribution system to meet current and future demand.

However, the city decided not to proceed with this costlier solution, and instead directed water managers to install Well #44 in an uncontaminated portion of the aquifer (directly across the river from the TCE plume). Though it is impossible to predict with current information, but there is a good possibility that Well #44's pumping may eventually smear the TCE plume across the width of the aquifer. If nothing is done to stop the spread of TCE, it could be only a matter of time before the entire southern aquifer is contaminated.

Although some argue that by cleaning up the TCE at its source (the old landfill), the TCE plume problem will disappear and make it unnecessary to protect downstream wells, it is known that even if the TCE source can be 100% contained at the landfill, it will be decades (perhaps many) before TCE that has already leaked out of the landfill will be flushed out of the system by natural through-flow. A report on the TCE problem identified that more than 100 gallons of liquid TCE have already passed through the aquifer, suggesting that much more remains at the source to continue contaminating the aquifer in future. If pools of liquid TCE exist somewhere in the aquifer, it could take centuries for natural flushing to clear the aquifer of this contaminant. This is a very safe prediction, based on examples of TCE contamination in many other aquifers.

The TCE problem, and its costly solutions, typify the logic behind the adage that "it is always cheaper to prevent ground water pollution than to clean it up after it has occurred."

Other evidence of degrading ground water quality in the lower Portneuf Valley is no less worrisome than such immediate threats to drinking water safety as TCE or PCE.

The concentration of chloride, a constituent of salt, has been gradually climbing in a number of Pocatello wells over 20 years for which data are available (Figure 14).

Two principal sources of chloride contamination are: 1) storm water and road runoff, and 2) septic systems. Either or both are believed to be responsible for the gradual increase in chloride with time. Septic leachate percolates from septic drain fields (along with nitrate, sulfate and other constituents), whereas storm water enters through parking lot drains, roadside ditches, and other road runoff control structures which promote infiltration.

By itself, salt is not a health hazard (other than its sodium constituent, for people on sodium-restricted diets). Rather, the threat posed by such runoff control structures is from other constituents of road and parking lot runoff, such as oil and grease, lead, and chemical spills along transportation corridors. The fact that salt is reaching the aquifer is the real cause for concern, because it indicates that pathways exist by which other, more harmful compounds can enter.

Both Pocatello and Chubbuck have numerous drain structures called dry wells which are designed to allow street and parking lot runoff to drain into the ground. No one knows how many of these control structures exist although some estimates range into the hundreds (City of Pocatello Engineering Dept., pers. comm., 1999).

Because these structures are located directly over the aquifer, because they promote large amounts of infiltration at point locations, and because the highly permeable Bonneville gravels underlie most of the valley floor, this source of potential contamination is considered a priority for future management action. The City of Pocatello is already taking steps in this direction through its new Storm Water Master Plan.

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 (Figure 15). 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.