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• What are Permeable Reactive Barriers (PRB)?A Permeable Reactive Barrier <br /> (PRB) is a barrier created below ground to clean up contaminated groundwater. A <br /> long, narrow trench is dug into the pathway of the contaminated groundwater <br /> flow. The trench is filled with reactive materials such as iron, limestone, carbon, <br /> or mulch. Once the water passes through the PRB, the chemical reaction known <br /> as denitrification treats the contaminated water. However, Ms. Thomas explains <br /> that this process can be invasive, and concerns about the disturbance to the land <br /> over time arise. Therefore, alternative carbon sources were needed. She states that <br /> the discovery of a liquid source, such as vegetable oil or soybean oil mixed with <br /> limestone (calcium carbonate for pH balance),proved to be effective in <br /> denitrifying the groundwater. <br /> • Liquid Injections PRB. The mixture of vegetable oil and limestone will be <br /> injected into the ground in several sections where it will spread creating a natural <br /> barrier. Ms. Thomas notes there are multiple factors that must be considered prior <br /> to selecting the injection site which include, the speed and direction of the <br /> groundwater flow, the type of soil to ensure the oil will spread evenly and a <br /> review of the tide as this will impact the effectiveness of the barriers. <br /> • 187 Lagoon Pond Road on Martha's Vineyard. Ms. Thomas explains an ongoing <br /> pilot study that entailed the nitrate profile. They performed a tracer test to <br /> establish the direction of groundwater flow and its speed. Soil borings were <br /> completed to ensure the soil type for optimal results. Additionally, a tidal <br /> influence study was conducted to determine any fluctuations in the water table <br /> elevation, which revealed none. Over the past two years, nitrogen levels have <br /> been reduced by 98%, with minor variations noted between seasons and depths. <br /> Furthermore, Ms. Thomas details secondary reaction concerns, such as an <br /> increase in iron, manganese, and arsenic. <br /> • Cost. Ms. Thomas is very explicit that the cost for each project depends on <br /> several factors, which will vary based on the site and the lack of information <br /> regarding the timeline for a PRB system. She adds that the town of Orleans has <br /> been using a PRB system for the past 8 to 9 years with no issues or needed <br /> reinjections. <br /> COMMISSIONERS COMMENTS <br /> Mr. Colombo inquires about the number of feet the barrier is and references the large <br /> number of injection sites noted in the slide. Ms. Thomas ensures that this is not a common <br /> practice but was completed to establish whether a 10-foot difference would suffice or 15 feet. <br /> She elaborates, and the study found that 10 feet was enough space between the injections to <br /> create a sufficient barrier. In addition, the injection sites will show no signs of any disturbance <br /> shortly after the treatment is complete. He follows up by requesting additional information <br /> regarding the varying soil porosity levels. Ms. Thomas is unable to provide any information <br /> regarding this matter. However, she does note that this topic is new, and studies regarding soil <br /> porosity are scarce. <br /> Mr. Cook asks for clarification on the distance between the injection site and the <br /> waterbody. Ms. Thomas states that the pilot site distance was 80 feet; however, the metals in the <br /> 2 <br />