No text on pollutant fate and transport would be complete without considering remediation of the contaminated system. Thus, each fate and transport chapter in this book (Chapters 5-9) will close with a section on remediation. Each environmental system discussed in this book presents a unique set of conditions, and thus problems, with respect to remediation. Issues specific to lakes include the hydraulic retention time, t0, introduced in Section 5.5.2, which is determined by the total volume of the lake and the effluent flow rate. A lake with a high retention time will not naturally or rapidly purge itself of pollutants in the water column, and thus the pollution will be present for extended times. The large volumes of water that most lakes contain makes infeasible any direct ex situ (outside of the lake) pump-and-treat process on the lakeshore. While soluble pollutants remain in the water column, other pollutant will concentrate in the sediments. In general, a longer residence time in the lake enables pollutant sorption to particles in the water column, such that the ultimate resting place of the pollution is in the lake sediments. This is especially true

Figure 5.13. A diagram of the shoreline and basins contained in Lake Hartwell (South Carolina). Mixing basins are delineated by solid rectangles.

for metals and hydrophobic pollutants. Concentration of the pollutants in the sediments has good and bad consequences, as we will see in the following paragraphs.

So, what can we do to remediate a polluted lake? First and foremost is to remove or eliminate the source of pollution to the lake. This is relatively easy for point sources such as the effluent from an industry, but becomes more complicated for effluent from a sewage treatment plant, since treated sewage has to be disposed of somewhere. The problem becomes even greater when we are talking about nonpoint sources that are not easily identified or eliminated (such as agriculture runoff) or are uncontrollable (such as leachate entering a lake from an abandoned landfill). Nonetheless, identification, isolation, and eliminate of the source of the pollution should be the first step in any remediation plan.

One of the most common remediation efforts undertaken for lakes is to treat eutrophication, the uncontrolled or excessive microbial and algal growth that results from the presence of nutrients from agricultural runoff and sewage treatment facilities. Even the slightest amount of phosphorus can induce eutrophication; thus, elimination of the source is important. In general, the remediation approach for eutrophied lakes is to eliminate the nutrients and let nature take its course (cleansing by removal through burial of nutrients in the sediments and by removing nutrients by outflow from the lake). This approach is usually the only viable option given the large scale of the problem (volume of water and hydraulic detention time in the lake).

Remediation of polluted sediments has also been the focus of many lake remediation efforts. We will discuss remediation strategies from the most to least complicated (and costly). One obvious, but expensive, way to remediate contaminated lake sediments is to simply remove the sediment by a technique referred to as dredging. There are many types of dredging, but all involve physically removing the sediment from the lake, placing it on a barge, and transporting it to a treatment or containment facility. But before this "sure fix" is adopted, one must consider the scale of the problem. The cost of dredging uncontaminated sediment is approximately $5 per cubic yard, but this cost increases to $15 to $20 per cubic yard for contaminated sediment (National Research Council, 1977). And what do you do with the contaminated sediment once you have removed it since it is not a hazardous waste? Treatment costs (thermal degradation, biochemical treatment, extraction) can range from $100 to $1000 per cubic yard (National Research Council, 1997). You start to see the scale of the problem created by this "sure fix" solution. Thus, dredging the sediments of a lake is usually not a viable option.

Another proposed option is to bioremediate the sediment by adding nutrients and pollutant-degrading microorganisms to the contaminated areas. While this sounds good in theory, it also presents several problems. For example, engineered microbes that effectively degrade pollutants have been raised in the lab, but it is unclear whether these same microbes can effectively compete in the real world of a lake sediment. Furthermore, addition of nutrients to a lake can be a problem in itself, since these nutrients will most certainly migrate to the water column and could promote eutrophication of the lake. In addition, metals do not degrade via microbial action. To date, no human-engineered bioremediation action has been conducted on a lake, although nature is always working through these very same pathways to degrade organic pollutants in lakes.

The next less costly, but also much more viable, approach is to "cap" the contaminated sediment by placing a layer of pollutant-free substrate on the area of interest in the lake. This can be achieved by strategically placing anything from synthetic substrate to clay to sand in the lake. This speeds the process of natural burial and can effectively prevent the contaminated sediments from resuspending and contacting the water column. Of course, considerations that must be made in using this technique include sediment depth from the water's surface, the surface area needing capping, and hydraulic flow rates in the lake that my uncover the contaminated sediments.

The least expensive, and in many cases the most practical, remediation is natural recovery resulting from a combination of biodegradation and burial of the contaminated sediment. Burial occurs by natural sedimentation of suspended lake particles and material that washes into the lake during storm events. As noted in Section 5.5.4, some lakes have natural sedimentation rates of centimeters per square meter per year that can relatively quickly bury a contaminated area of the lake. This remediation approach may not be appropriate for every lake, but does prevent the release of pollutants into the water column during dredging activities. These releases can be significant, as up to 50% of the sorbed mass of pollutant can be released in a period of hours to days (Dunnivant et al., 2005). Sorption and desorption processes were discussed in Sections 3.3 and 3.4.

All of the remediation approaches discussed above must also include some form of long-term monitoring project to document the extent of pollutant occurrence in the lake, future releases of pollutants from the dredged or in-place sediments, and the overall effectiveness of the remediation effort. The extent of this monitoring can range from a few months to decades.

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