Congressional District # 07
MORRIS ARSENIC DUMPEPA ID# MND980792287
Last Updated: November, 2011
The Morris Arsenic Dump site is located in west-central Minnesota near the intersection of Minnesota State Highway 28 and U.S. Highway 59, approximately one mile northeast of the city of Morris. The site is the location of an old gravel pit that lies just south of the highway intersection. To the southwest and west are several trailer homes and a small commercial structure, and to the east across U.S. Highway 59 is a state-owned gravel pit that is partially filled with water. A single residence and the Pomme de Terre River are located further east of the state-owned gravel pit. The city’s municipal wells are about 1 mile southwest of the site.
In the early 1940s, Stevens County reportedly had buried an estimated 1,500 pounds of surplus arsenic-based grasshopper pesticide (or “bait”) in the old gravel pit. In 1978-1979, the U.S. Highway 59 bypass was constructed through the old gravel pit location and in the general area of the grasshopper bait burial site. In 1980, the Minnesota Pollution Control Agency (MPCA) received a report that grasshopper bait was buried near the new bypass and MPCA proceeded to conduct a limited soil sampling event in an attempt to find the arsenic deposit. MPCA reportedly did not find the arsenic deposit and requested technical assistance from EPA. In response, in late 1981, EPA installed and sampled 12 groundwater monitoring wells and also took soil samples at the site. Although EPA found no arsenic-contaminated soils, the groundwater samples showed arsenic levels to be “above background levels” and one of the (unfiltered) water samples had a very high (over 1000 micrograms/liter or parts per billion (ppb)) arsenic reading. The site was placed on the Superfund National Priorities List (NPL) in 1983.
EPA conducted a remedial investigation at the site in 1984 (see "Cleanup Progress" section, below), but no feasibility study was produced for the site because no adverse human health or ecological impacts were noted. EPA issued a proposed plan for No Action for the site in early 1985 and we signed a Record of Decision (ROD) in August 1985 that selected the No Action alternative. EPA selected the No Action decision because the arsenic concentrations in the soil samples were below U.S. background concentrations (0.1 to 194 milligrams/kg or parts per million (ppm)) and groundwater arsenic levels were well below the then Safe Drinking Water Act Maximum Contaminant Level (MCL) of 50 ppb. (The current MCL for arsenic is 10 ppb.) The 1000+ ppb arsenic reading was not duplicated in subsequent groundwater samples; also, since the orginal samples had not been field-filtered although the water was turbid, the high arsenic reading was suspect. The ROD did not require further groundwater monitoring or the conduct of five-year reviews at the site. No institutional controls (ICs) were proposed for implementation at the site.
The Morris Arsenic Dump site was deleted from the NPL in 1986.
Site ResponsibilityThis site was addressed through federal actions.
Threats and Contaminants
Arsenic was the only potential contaminant of concern detected in site soil and groundwater. It is a carcinogen and also can cause non-cancerous illnesses or death.
The levels of arsenic attributable to the site do not pose a health hazard to the public. It is common to find arsenic in the soil and groundwater in the glacial deposits in the western Minnesota area; the arsenic is derived from naturally occurring geologic sources as well as from the wide use of the grasshopper bait (pesticide) in the 1930s/1940s. The Minnesota Department of Health routinely recommends that any new drinking water wells in the western area of the state be sampled for arsenic and has published a fact sheet concerning this issue. It is available on the state's Internet web-site.
EPA conducted a remedial investigation (RI) at the site in 1984 to try to find the reported arsenic deposit and to determine whether any arsenic had contaminated the groundwater aquifer. The sampling results demonstrated, however, that construction of the highway bypass through the center of the gravel pit location had made the discovery of the exact location of the buried grasshopper bait rather unlikely. A conclusion was made in the RI Report that the arsenic deposit may have been mechanically dispersed during the highway construction since topsoil cleared from the site for roadbed preparation was later spread along the side slopes of the road.
Below is a discussion of the testing approaches taken at the site during the RI:
EPA first conducted geophysical testing to try to delineate the edges of the gravel quarry and to find the grasshopper bait deposit. An electromagnetic conductivity meter was used to measure ground conductivity values for the 0-3 meter depth and 3-6 meter depth ranges of the unit. Aerial photos were used to determine the approximate location of the old gravel pit and to focus the geophysical testing in that area. The resultant meter readings helped EPA to determine the approximate boundaries of the gravel pit, but meter readings did not identify a potential burial area for the grasshopper bait. Readings did help us to determine the approximate depth of the gravel pit and that it is now filled with a mixture of clays, silts, and organic material.
EPA next hand-augured and/or drilled a total of 47 soil borings into the western side of the old gravel pit (where the grasshopper bait may have been buried) and sampled the soil at discrete depths. The borings confirmed the western limits of the gravel pit, with clean sand and gravels encountered outside the pit and fill material within the pit that consisted of organic material (decomposing manure and wood chips), topsoil, sand, clays, silts, and gravel plus assorted metals and trash. The fill material was about 6-7 feet thick and generally extended down to the water table, although the native sand and gravel beneath the fill appeared to have been reworked (disturbed) so that the bottom of the pit could not be determined using the soil boring sampling results.
Arsenic levels in surface soil samples averaged about 7 mg/kg ("parts per million" or ppm). EPA identified several locations with elevated arsenic levels within the old gravel pit area that occurred in a thin, black-stained soil layer right at the groundwater table capillary fringe (about 3 to 8 feet below ground surface). The total area of the potentially-impacted zone was about 5,000 square feet. Here, the arsenic levels were generally in the 20 ppm to 40 ppm range, although 6 samples (out of the more than 100 analyzed) exceeded 50 ppm and one of these samples was at 104 ppm at 8 feet below ground surface. Research showed that site-area “background” or naturally-occurring soil arsenic levels range from 3 to 14 ppm, although values as high as 100 ppm or more have been reported in Minnesota.
The soil sampling effort did not find the buried grasshopper bait deposit. Based on the 1500 pound estimate from the county, and research that shows the bait to likely contain between 1% and 10% arsenic (10,000 ppm to 100,000 ppm), a total of about 15 to 150 pounds of arsenic may have been buried at the site. Based on the RI soil sampling results, EPA calculated that the total amount of arsenic present in the thin, 5,000 square-foot soil mass (at levels above background) was about 90 pounds, which is within the estimated range of the amount of arsenic that may have been buried at the site.
EPA sampled the 12 existing monitor wells during the RI. Groundwater samples were field-filtered (due to very high turbidity encountered and because it was the standard practice at the time) and results indicated that dissolved arsenic levels ranged from <1 to 8 ppb, averaging about 4 ppb. Groundwater samples were taken from some of the soil borings as well and results indicated similar dissolved arsenic levels as the monitoring well samples. Research showed that “background” arsenic levels in the groundwater of west-central Minnesota (the site area) to range from < 0.5 ppb to 130 ppb and to average about 3 ppb. Since the high pre-RI groundwater arsenic levels were not duplicated and no groundwater contaminant “plume” was identified, EPA and MPCA decided to install no additional monitoring wells at the site.
EPA sampled five residential wells around the site, four of which were hydraulically upgradient and one sidegradient. The arsenic levels measured in the private well water samples were comparable to the levels measured in the monitoring wells and the area background level. The eastern residential well was sampled twice (June and July 1984) and arsenic was reported at about 5 ppb and about 8 ppb. These levels were below the current MCL for arsenic, which is 10 ppb.
The groundwater aquifer used by area residences and municipalities as a drinking water source is a thick, poorly-sorted glacial deposit consisting of clayey sand and gravels. The state-owned gravel pit and the Pomme de Terre River was shown to be hydraulically downgradient of the site, and the eastern residence to be side gradient, based on water level measurements taken during the RI.
EPA took 5 samples of the pond water and sediment in the state gravel pit across the highway. Results showed arsenic levels in the water and sediment to be comparable to background groundwater and soil arsenic levels, respectively.
In summary, EPA determined that the buried grasshopper bait could have been dispersed by mechanical means within the old gravel pit area and covered by fill materials when the highway bypass was built. Although soil areas with slightly elevated arsenic levels were found, no soil samples approaching the original estimated arsenic concentration in the grasshopper bait were noted. Mass balance calculations indicate that the impacted 5000 square-foot soil area likely contains all the arsenic originally buried by the county. Measured off-site groundwater, surface water, river water, and sediment arsenic levels indicated that the arsenic is not leaching from the site because the all the results correlate to background arsenic levels in the state.
Although arsenic was detected at the site, the levels do not pose a health hazard to the public. Site arsenic levels in the soil and groundwater are within the range of naturally-occurring levels in Minnesota. Therefore, no cleanup actions are necessary. EPA issued a Record of Decision for No Action at the site in August 1985. The site was deleted from the National Priorities List on March 7, 1986.
Success StoryEPA quickly responded to a potential health threat.
Property ReuseThe site is mostly used as a road bed for the U.S. Highway 59 Bypass.
ContactsRemedial Project Manager, U.S. EPA
gladys beard (email@example.com)
Community Involvement Coordinator, U.S. EPA
AliasesMORRIS ARSENIC DUMP SITE