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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.

This write-up summarizes the journal article Geochemistry of Extremely Alkaline (pH>12) Ground Water in Slag-Fill Aquifers (2005) written by George S. Roadcap, Walton R. Kelly, and Craig M. Bethke.  The case study provides covers treatment techniques for an impaired aquifer, ultimately recommending air sparging as the best remediation alternative, of the four considered, for the remediation of high pH ground water under the conditions specified in the article.

Four (4) remediation schemes aimed at reducing the high pH waters were examined in this article, which are as follows.
•    Carbon Dioxide (CO2) sparging
•    Air sparging
•    Hydrochloric acid (HCl) addition
•    Dolomite (CaMg(CO3)2) addition

The article thoroughly summarizes the subsurface conditions in the Lake Calumet region near Chicago, IL, an area with several active and former steel mills.  The article serves as a valuable resource for people involved in remediating high pH ground and surface waters, as it is one of only a few articles covering this topic.  Although slag, a by-product of steel manufacturing processes, caused the poor water conditions in this location, the geochemistry and remediation aspects of this article are universally applicable to sites with high pH, and high concentrations of TDS, Iron and Ammonia.

A wide variety of industrial processes, in addition to steel manufacturing, may contribute to extremely alkaline waters.  As such, there may be many more sites that require remediation of the types included in this discussion.  Data collected from four (4) sites near Lake Calumet were analyzed in the authors’ discussion.  The sampling locations include two (2) springs (sites 2 and 7), one (1) shallow pond (site 3), and one (1) test pit (site 4), created for the purpose of collecting data steel slag samples (Roadcap, Kelly, and Bethke, 2005, Page 807).

The authors explained the methods used to collect and analyze the samples collected from the 4 locations listed above.  Conductivity, pH, temperature, and DO were measured in the field after proper calibration (Roadcap, Kelly, and Bethke, 2005, Page 808).  Sampled collected in the field were analyzed by an IL laboratory, and included several major and minor elements (Roadcap, Kelly, and Bethke, 2005, Table 1, Page 809).  X-Ray diffraction (XRD) was used to classify the mineral stratification in site 4.  Additionally, geochemical modeling was used to obtain other pertinent information such as dissolved gases fugacity (Roadcap, Kelly, and Bethke, 2005, Page 808).

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.