Remediation technologies for arsenic contamination at Deloro Mine

Deloro Mine Remediation

Evaluation of Remediation Technologies for Arsenic contamination at Deloro Mine Eastern Ontario Canada

Site

Deloro is a small town of 180 people located in eastern Ontario. For many years, the residents of this town were unknowingly breathing arsenic contaminated air and drinking contaminated water. Recent legal actions have brought this tiny town to the forefront. The pollution stemmed from a 280-hectare mine located on the eastern edge of town. Sampling sites from 18 sites around the village averaged an arsenic level of 723 ppm, with one location as high as 5,500 ppm (McAndrew).

Two likely sources of contamination have been sited as potential culprits. The first is arsenic dust emitted from refinery smoke stack the settled on the soil. However, the refinery closed in 1961. The second potential source of contamination is from piles of mine waste, known as "tailings." These tailings piles are now covered with rocks to prevent dust from blowing into the town and continuing to increase the contamination levels (McAndrew). Arsenic is a by-product of the mining industry.

Scope of the Project

Removal of arsenic contaminated compounds was not feasible. Therefore, the Ontario Ministry of the environment built an Arsenic treatment plant to treat contaminated ground water. This plant is capable of removing 99.5% of the arsenic from the contaminated water that it treats (Ministry of the Environment, 2005). In addition, they have instituted an aggressive water monitoring project and associated laboratory to conduct on-site testing and analysis.

Remediation of the Deloro Mine site has been a multi=phased project. Decontaminating the old arsenic mines was not feasible. Therefore, they simply closed off the entrances in such as manner that arsenic would not continue to be entrained in the airstream, causing continued contamination. They demolished buildings and disposed of them in hazardous landfills. They capped eight hectares of tailings by placing 1/2 meter of limestone on top of them. This would help prevent arsenic contained in them from becoming entrained in the airstream or eroding and entering the water stream. The entire site has been fenced to prevent trespassers from entering the site and carrying hazardous wastes away on their clothing (Ministry of the Environment, 2005).

Groundwater and surface water will be diverted away from contaminated materials through the use of a passive interceptor well network (Ministry of the Environment, 2005). This waster will flow through a groundwater collection and treatment facility already in existence. The treatment plant is planned to operate indefinitely (Ministry of the Environment, 2005).

Contamination of the Moira River remains a key concern in terms of reducing threats to humans. The Moira River flows through the center of the site, presenting one of the most dangerous threats to human health. Arsenic enters the Moira River through erosion of contaminated soil, or from airborne sources. The river then carries the arsenic downstream and deposits it. The River empties into the Bay of Quinte at Belleville. Contamination of the Moira River pollutes major lakes along the way. Efforts at remediation of the Moira River have succeeded in reducing the arsenic in the river by 80% (Ministry of the Environment, 2005).

However, when one considers the extent of the contamination, this waterway still represents a considerable risk to humans.

Another area of major concern is the Young's Creek Area, which is part of a significant wetlands. This is a low-water flow wetlands where arsenic compounds are deposited in the sediment. Arsenic levels in this area affect local plants and animals, in addition to the human risks associated with contact with the waterway. Remediation in this area involves evacuating the water from the area and removing contaminated sediments. These sediments will be stored permanently in a containment vessel. Sediments will be replaced with clean fill and a wetland recovery project will be initiated.

Geology/Hydrogeology of the Deloro Site

The Complex geology of the Deloro area accounts for is development as a successful mining endeavor. It also contributes to the difficulties associated with the remediation process. In general, the surface of the ground slopes to the South and to the East towards the Moira River. This slope is largely responsible for the erosion of contaminated soils flowing into the Moira River. Elevations range from 210 meter above sea level to 185 meters about sea level (CH2MHILL, p. 3-2). The Moira River represents the low-lying area of the river. Mining activities have significantly altered the topography of the area, increasing the rate and are of run-off into the Moira River.

The Deloro Site contains two different forms of bedrock. Precambrian metasedimentary and metavolcanic rock comprise a majority of the bedrock under the site. However, there is a small area of Peleozoic Limestone and Shale along the eastern edge of the property (CH2MHILL, p. 3-2). Young's Creek lies in a depression in the bedrock that is flanked by outcroppings and exposed bedrock. Much of the exposed bedrock lies on the north end of the Moira River (CH2MHILL, p. 3-2). Exposed bedrock demonstrates excessive weathering. Much of the exposed bedrock is pink granite of medium grain (CH2MHILL, p. 3-2).

Bedrock at the Deloro Site is overburdened by both natural and man-made components. Overburden includes clay fill, building rubble, tailings and slag. Natural overburden includes silty clay, silty peat, and silty sand (CH2MHILL, p. 3-2). Overburden is generally thin, averaging 3 meters in most areas, but can go up to 9 meters at times (CH2MHILL, p. 3-2). Native soils primarily reside in areas of low topography.

This topography allows for constant movement and shifting of overburden. Rainy seasons create excessive water flows, which travel down slopes to the low-lying areas and into the waterways. These areas of low topography are of particular concern, as they tend to accumulate deposits of contaminants.

Groundwater and surface water in the area are altered by man-made structures placed by the mining company. Water travels through the area by means of the overburden and the bedrock. In the area of the main mine, water flows primarily through bedrock (CH2MHILL, p. 3-2).Groundwater flows in a generally southeasterly direction. Water flows towards the major waterways such as the Moira River and Young's Creek (CH2MHILL, p. 3-2). Bedrock in the area is complex, providing many depressions, faults, and folds. These are generally shallow and near the surface (CH2MHILL, p. 3-2). The bedrock consists of numerous dykes, sills, xenoliths and highly mineralized areas (CH2MHILL, p. 3-2). These structures have been altered by mining activities, such as blasting.

The Deloro Site is located within the Moira Watershed, which discharges into the Bay of Quinte. The Moira River flows through three lakes: Wolf, Moira, and Stoco Lake. Many of the mining activities took place upstream from Moira Lake. Most of the waterways associated with the Moira River are shallow. Bend Bay averages only 1 Meter in depth. Moira Lake is a widening of the Moira River that averages 3 M. In depth (CH2MHILL, p. 3-2). The Moira River provides the primary inflow and outflow of these structures. The river water is well-mixed during the spring and fall, but can developed thermal layers during the summer in deeper portions of the lake.

The geology of the area creates conditions that promote the formation of shallow water tables. Water is generally slow moving. The degree of mixing of the major wateways promotes homogeneity of contaminants. There may be some settling of arsenic in sediments during the summer as thermal layers develop. The Young's Creek area is particularly at risk for developing higher concentrations of sedimentary arsenic due to the slower flow pattern. The geology and hydrogeology of the Deloro Site contribute to the difficulties associated with remediation.

Motivation for the Thesis

The philosophy of the Deloro Mine Site Remediation differs from other hazardous sites. The number of methods of cross contamination at the site represents the most difficult aspect of remediation efforts at the Site. It would be unrealistic to consider approaching the contamination issues at this site using standard Phase I and Phase II protocols. One has to change philosophy in a situation such as this. Instead of looking at the site as an area that can be decontaminated and deemed "clean" as in most remediation projects, the Deloro Mine Site will never be able to be considered "clean." The best that can be achieved in this case is ongoing management that minimizes the risk to humans as much as possible. This is the approach that has been taken by the Ontario Ministry of the Environment. Monitoring and containment projects will be ongoing at the site.

Contamination at the Deloro Mine Site represents one of the greatest threats to humans. My primary motivation in this thesis is a desire to help protect the human lives that it touches. The scale of the contamination at the site exceeds what one expects at a hazardous site. The Deloro site is not a "typical" remediation project. The site will require continuous monitoring for many years to come. Seldom does one get the chance to work at a site that is as complex as this one. The Deloro Site represents a large scale natural disaster. It could be considered a worse-case scenario in remediation projects. There are many concerns on this site that are not present in other remediation projects. Experience gained on this site will be valuable in any project with which I may become involved in the future.

2.0 Arsenic

Chemistry

Although Arsenic is a basic element, it often does not occur in pure elemental form. Arsenic compounds comprise a majority of the arsenic found on earth. The most common arsenic compounds are sulfides, arsenopyrite, orpiment, realgar, lollingite, and tennantite (USEPA, 2006). Arsenic is often found in silver ore, dycrasite, barite, cinnabar and nickeline. Silver and nickeline associated arsenic are largely responsible for what is found at the Deloro Mine. Arsenic can form into several chemical forms. It can appear trigonal or in an orthorhombic form. It is highly versatile in its ability to form compounds.

Safe Drinking Water Standards are based on total arsenic, comprising both inorganic and organic sources. The U.S. has one of the toughest standards in the world, regarding arsenic levels in water. The maximum total arsenic content of water cannot exceed 10 ppb (USEPA, 2006). This standard was lowered from 50 ppb in 1975. The Canadian maximum arsenic level in drinking water is 0.010 mg/L (British Columbia Ministry of Environment). Background levels of arsenic are generally low in Canada and the U.S. Much of the arsenic occurring in water is the result of human activity, rather than natural processes.

Health Effects

Arsenic enters the body by several mechanisms. It can stem from food grown in contaminated soil, or it can be absorbed through the skin. It can also be inhaled. Symptoms of arsenic poisoning can be chronic or acute. Arsenic poisoning at low concentrations can produce nausea, headaches, an odd skin color, anorexia, and white lines across the fingernails (McAndrew). At higher concentrations, arsenic can produce vomiting, renal failure, and death (McAndrew). Long-term exposure to high arsenic concentrations has been implicated in many different form of cancer. Arsenic exposure poses a serious health risk to humans.

Sources

Arsenic is a naturally occurring element. Almost any waterway will contain at least some concentration of arsenic. These background level are generally low, but can be elevated in areas where water passes over exposed arsenic ore outcroppings. Common background levels from natural sources range between 0.001 to 0.002 mg/L in the U.S. And Canada (British Columbia Ministry of Environment).

Arsenic levels around the Deloro Mine have been found as high as 700-5,000+ ppm (McAndrews). Levels in this range cannot be attributed to naturally occurring sources. Levels this high indicate a manmade source. In this case, mining activities exposed large sources of arsenic containing ore. Rains washed arsenic compounds into the waterways, where it made its way into the groundwater. The site also contained an arsenic refinery where arsenic was separated from the ore and turned into an arsenic containing sludge. Arsenic containinated tailings piles were also washed into the waterways. This type of activity is the most common source of arsenic contamination in waterways.

Motility in Groundwater

Arsenic is highly mobile in groundwater. Aside from ease in traveling in slow moving streams. It is highly reactive and when settled in sediments can be re-released into the water column by chemical or physical activity. Arsenic released into surface water can result in higher concentrations in ground water. Groundwater with higher flow rates is especially prone to highly mobile forms of arsenics. Organic forms of arsenic can release highly mobile elemental arsenic into the water by means of oxidation and reduction reactions.

Arsenic's ability to remain highly mobile in ground water is a key concern in remediation projects. One of the key factors in the potential for high arsenic levels in groundwater is the amount of material available to be introduced. In many projects, the amount of material that can be entered into the system is limited. However, at Deloro, there is the potential for contamination for many years due to the amount of material available to be introduced into the system.