Environmental Engineering Consulting

The authors make important notice of the effect of oxygen on the water chemistry of slag-fill aquifers by observing that the shallow pond-sampling site (site 3) that had high DO concentrations also had high nitrate/nitrite relative to ammonia (Roadcap, Kelly, and Bethke, 2005, Page 811). In this instance, the results of the sampling activity are intuitive since obviously available nitrogen will bond with oxygen to form either NO3/NO2, prohibiting, to least some degree, the formation of ammonia (NH3).  Once the topic ground water had been thoroughly covered, its mixing with surface waters were covered at length.

Sampling at site 7, a spring mixing with two (2) surface water inflows, and downstream demonstrated that pH decreased rather quickly as it was mixed with other surface waters and the atmosphere.  As noted previously in the article, calcite forms in the spring’s discharge ditch under conditions that would prohibit its formation in neutral pH waters.  At this site, calcite forms due to the introduction of carbon dioxide from the atmosphere and its reaction with Ca2+ in the spring (Roadcap, Kelly, and Bethke, 2005, Page 814).  The concentration of most cations increased along the ditch’s flowpath, pH decreased.  These results are also somewhat intuitive since metals have increased solubility in lower pH waters.

Since the geochemistry described in detail in the article and summarized above demonstrates that the introduction of CO2 facilitates pH lowering, CO2 sparging seems like a very good option for remediating the stream.  Air sparging and hydrochloric acid addition are two other commonly used treatments, and were included this article.  Dolomite was available locally and had the chemical potential to reduce pH as well, which is why it was considered.

The authors wisely considered each treatment’s effect on aquatic toxicity.  The untreated water had a 100% organism mortality rate; waters with pH effectively lowered by treatment with CO2 sparging and hydrochloric acid addition yielded 30% to 40% mortality rates, and air sparging yielded a 10% mortality rate (Roadcap, Kelly, and Bethke, 2005, Page 815).

CO2 sparging was most successful at achieving the expressed purposed of the remediation, pH reduction, of the four (4) attempted.  It lowered pH very quickly; however, I agree with the authors that it is not an acceptable alternative because of its relative ineffectiveness at lowering organism mortality.  Hydrochloric acid addition is eliminated from the list of possible alternatives for the same reason, while dolomite addition did not adequately reduce the pH.

REFERENCES

Roadcap, George S., Kelley, Walton R., and Benthke, Craig M. 2005. Geochemistry of Extremely Alkaline (pH > 12) Ground Water in Slag-Fill Aquifers. GROUNDWATER, 43(6), 806-816.

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I am a graduate of the University of South Carolina’s College of Engineering and Information Technology with a Bachelor of Science in Civil Engineering with a special interest in environmental engineering.

I am a civil environmental engineer working in the environmental engineering consulting field.  My primary areas of expertise are air quality permitting, solid management including RCRA regulation, and water resources topics including TMDL Implementation.  I hope to use this site as a means of sharing information and learning from others with similar interests.