Groundwater Pollution Of The Black Lake

Groundwater is an essential source to meet the ecosystem’s water demands. It is known to make up 20% of the world’s freshwater supply and approximately 51% of the drinking water supplied in the United States. The groundwaters sustain the rivers, lakes and other aquifers, while also playing a key role in providing moisture to the soil.

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While groundwater pollution is invisible to the eye, it is the most difficult to clean up. Contaminants can easily be transported through the aquifer via processes like diffusion, adsorption, percolating through the adjacent soil, into the linked water reservoirs and, even travelling through roots of certain trees and plants. Thus, the possible fate of groundwater contamination can immensely decrease the quality of the ecosystem around it, in turn posing a great threat to living beings.

The Black Lake Grocery is a convenience store and active gas station, located in Thurston County, Washington. It is situated north-west of the shore of the Black Lake. A potential issue arose in 1989, concerning groundwater and soil contamination of petroleum hydrocarbons (gasoline). The major contaminants observed were TPH-G (Total Petroleum Hydrocarbons – Gasoline) and BTEX (Benzene, Toluene, Ethylbenzene, Xylene). The officials were further concerned with the potential transport of chemicals into the surrounding land, and especially the Black Lake.

In 1992, a research conducted by the Washington State Department of Ecology ranked this site as a ‘2’ on its Ecology Hazards List (rankings ranging from 1 to 5, 1 being the topmost on the priority list). To prevent further hazard, a portion of the contaminated soil and all underground petroleum-storage tanks were removed. In 2004, treatment walls were installed as a passive barrier to the Black Lake, with scheduled groundwater sampling and testing every 18 months. Sample testing was performed to determine concentration of the contaminants.

The groundwater elevation was divided into 3 zones for analysis:

  • Zone 1 – the upper portion of the plume,
  • Zone 2 – groundwater immediately upgradient,
  • Zone 3 – groundwater immediately downgradient of the treatment wall.

According to the ‘Cleanup Action Plan’ (CAP) and ‘MTCA Method A Cleanup Level’ (MTCA), the limit of m,p-xylene in the groundwater samples is 20 and 1000 µg/L or parts per billion (ppb) respectively, to be below the hazard limit. The latest monitoring results published by the Washington State Department of Ecology in June 2017 concluded that the m,p-xylene concentration highly exceeded the established cleanup levels in all three zones. Following were the observed results – Zone 1 – 560 ppb, Zone 2 – 9270 ppb and, Zone 3 – 382 ppb (north of the downgradient) and 1610 ppb (at the downgradient). 1,4-Dimethylbenzene, or commonly known as para-Xylene [Chemical formula: C6H4(CH3)2; C8H10] is an isomer of xylene.

It is a colourless gas that occurs naturally in petroleum and is commonly present in automobile exhaust and industrial activities. It is highly flammable with a flash point of 27 deg Celsius. It doesn’t readily dissolve in water and can quickly evaporate into the air. It is rapidly absorbed by human lungs after breathing air containing the chemical. 50-75% of it is retained while the rest is exhaled immediately. Physical exercise is said to increase p-xylene absorption in the system. p-Xylene breakdown occurs in the liver of human beings resulting in by-products, particularly methylbenzaldehye which is harmful for the lungs.

Xylene, in general, as a chemical has not yet been classified as a carcinogen, nevertheless posing a great threat to the human body and other organisms. Human exposure to low levels of p-xylene can cause dry skin, redness in eyes, dizziness, headache and nausea. In extreme conditions, it can also impair one’s nervous system and cause pulmonary edema. The threshold limit value, i.e. exposure above which it gets hazardous is 100 ppm. The bioconcentration factor of p-xylene is 95 according to the EPA. A few cases have made known concerns regarding the effect of p-xylene contamination in water on shrimp, eels and manila clams.

Due to insolubility of p-xylene in water and having a density lower than water (rho = 0.861 g/ml), p-xylene simply rises to the surface of the water reservoir. With a vapour pressure of 9 mmHg (20C), it volatilizes into the air where it is subjected to photo-enhanced oxidation and breaks down into less harmful chemicals. This photodegradation in air enhances rapidly in presence of nitric oxide. Meanwhile, biodegradation of -xylene in water is very slow and only develops under iron-reducing conditions. Even below the land surface, the chemical is expected to breakdown by the organisms present in the soil into less harmful chemicals, but it is a very long process.

The soil adsorption coefficient (Koc) for p-xylene is approximately 254-540 L/kg. This indicates ease in chemical mobility, and hence justifies the ability to percolate through soil particles. The p-xylene molecules once released, volatilize through the soil, into the atmosphere. In case of contamination at the groundwater level, the volatilization of the chemical is hindered, and it gets leached into the ground. Similarly, the p-xylene in the Black Lake Grocery site is expected to percolate through the soil particles and undergo volatilization in the region above the downgradient of the treatment wall. Due to the gradient path of the groundwater stream, the contaminant will possibly flow into the Black Lake across the treatment wall, where it should volatilize to the water surface and into the air and photo-oxidize.

18 March 2020

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