According to the EPA website, "When this crisis occurred, Coast Guard and EPA granted BP [British Petroleum] authorization to use an approved dispersant on oil present on the surface of the water in an effort mitigate the impact of the spill." Then, the EPA and the U.S. Coast Guard authorized BP to use dispersants underwater, at the source of the Deepwater Horizon leak. Preliminary testing results indicate that subsurface use of the dispersant is effective at reducing the amount of oil from reaching the surface - and can do so with the use of less dispersant than is needed when the oil does reach the surface.
Concerned about the potential adverse effects of dispersant use below the surface, the EPA enacted an aggressive dispersant monitoring plan to be implemented by BP and that findings are routinely and carefully analyzed to ensure toxicity data is collected that may indicate any significant effects on aquatic life in the region. The good news so far is that the decreased size of the oil droplets is a good indication that, so far, the dispersant is effective.
An important parameter of these subsurface water quality studies is the monitoring of dissolved oxygen levels. Dissolved oxygen (DO) analysis measures the amount of gaseous oxygen (O2) dissolved in the water. Adequate dissolved oxygen is necessary for good water quality that is conducive to marine life. Normal ranges for DO in the Gulf area are 4 milligrams per liter (mg/l). The lower the concentration of dissolved oxygen, the greater the stress is on aquatic life. The evaluation criteria to determine further use of subsea dispersant include DO levels that are less than 2mg/l and the results of toxicity tests.
Concern initially arose when dissolved oxygen levels appeared low when first measured with one device. Fueled by the need to corroborate these findings, the EPA used an Extech dissolved oxygen meter for increased accuracy. According to the EPA, "In order to conduct a more thorough analysis, more sensitive equipment was then employed, called an Extech Probe. The subsequent dissolved oxygen readings from the Extech Probe indicate that dissolved oxygen levels are within the normal range." So far, using the Extech dissolved oxygen meter, DO measurements continue to remain in the normal range. Read the EPA's coverage for additional details.
Findings related to dissolved oxygen, water conductivity and temperature, oil droplet size, and other physical characteristics will serve as indicators of the effectiveness of BP's efforts to manage the oil spill using subsurface dispersants. If any negative impacts on the environment or public health are identified, the EPA can use that data to direct BP to discontinue the use of this dispersant method.