Presentation on theme: "Background Site of a former Industrial Waste Treatment Plant Contaminants accumulated in sludge drying beds and surge pond, they subsequently leaked into."— Presentation transcript:
Background Site of a former Industrial Waste Treatment Plant Contaminants accumulated in sludge drying beds and surge pond, they subsequently leaked into soil. Contaminants; VOC’s, SVOC’s and metals Condemned in 1987 PENSACOLA BAY
Site Geography/Geology/Hydrology The soil is made up generally of fine to medium sand, with a clay layer between 40 and 60 feet. There is an aquifer which has three zones of flow associated with the three soil layers. Overall flow is towards Pensacola Bay. Pensacola bay is down gradient. PENSACOLA BAY
Initial Solution The waste from the sludge drying beds and surge pond was solidified by dewatering and removed. Underlying soil was excavated to ground water level and backfilled. A pump and treat system was installed in Including 7 extraction wells and 19 monitoring wells.
Review The efficiency of the pump and treat scheme was evaluated in the mid nineties. The levels of most contaminants were no longer a major problem. Natural attenuation processes appear to be having an effect. Chlorinated ethenes, mainly TCE (trichloroethene) and chlorinated benzenes remained in high concentrations. Investigation to find if Natural attenuation oriented methods would be cheaper and more efficient.
Natural attenuation The efficiency of natural attenuation processes are very dependant on ground water systems and chemistry. Biodegradation was shown to be effective for chlorinated ethenes. Their levels being sufficiently reduced before their plume reaches Pensacola Bay. Natural attenuation was shown not to be effective for chlorinated benzenes in the anoxic region. Levels were reduced mainly due to dilution, though biodegradation effectual in the oxic region. No traces of chlorinated benzenes have been found in Pensacola Bay
Revised solution A solution taking advantage of the natural attenuation processes was proposed. The injection of an intensive in-situ chemical oxidising agent (Fenton’s Reagent) to reduce contaminant levels at the source. Having reduced source contaminants,the remaining lower levels can be reduced to safe levels by natural attenuation processes. A monitoring scheme was still required to monitor how effectual the processes are being.
The Practice I Ultimately two injections of Fenton’s Reagent were required. After the first injection TCE levels dropped from 3,600µg/L to 460µg/L. Large quantities of iron in the soil increased the rate of the reaction, reducing the zone of influence of the reagent. A second injection was employed with phosphoric acid to slow down the reaction and let it act of a larger area. After the second injection levels of TCE fell to 180 µg/L, and no further treatment was required.
The Practice II Pump and treat method – 20 years in total. Estimated 3 to 5 years for levels of TCE to fall below required levels – an injection of oxygen releasing compound employed to create an oxic environment to promote the biodegradation of chlorinated benzenes levels of chlorinated benzenes reduced by about 97%.
Costs Pump and treat method Annual Monitoring $100,000Annual operation $70,000 Revised Treatment Annual Monitoring $100,000Total Treatment $250,000 At the time when the revised treatment was introduced Pump and treat would have continued for a further 10 years costing $1,700,000 The revised method would have continued for 5 years max costing $750,000