1. TIME CRITICAL REMOVAL ACTION IN RESPONSE TO CONTAMINATED SOILS ALONG THE EASTSIDE DITCH NEAR DEER LODGE, MONTANA G.S. Vandeberg1, D.J. Dollhopf1, D.R. Neuman1, P.D. Smith2, W. Bluck3 and S. Brown4 1Reclamation Research Unit, Montana State University, Bozeman, MT. 2CH2MHill, Boise, ID. 3W.B.E. Inc., Helena, MT. 4U.S. Environmental Protection Agency, Helena, MT.

2. INTRODUCTION
The U.S. EPA issued a Unilateral Administrative Order to ARCO in May, 1999 for the Deer Lodge Valley Irrigated Lands Time Critical Removal Action. This order concerned lands adjacent to the Eastside irrigation ditch near Deer Lodge, Montana (Figure 1). These lands are within the Clark Fork River Operable Unit of the Milltown Reservoir Sediments NPL site, and have been impacted by historic irrigation which deposited elevated levels of arsenic, cadmium, copper, lead and zinc from the Clark Fork River. Many of these lands exhibit bare areas and poor vegetation coverage due to both elevated metal concentrations and depressed pH. The contamination originated from mining, milling and smelting activities in Butte and Anaconda. The Eastside ditch was partially abandoned in the 1970's. Adjacent lands are currently used for residential and agricultural use.

3. INTRODUCTION CONT.
The preferred action for the residential properties listed in the order is the excavation and replacement of soils (up to 18 inch depth) whose arsenic values exceed the trigger value of 120 mg/kg (95% lower confidence interval of the human health risk-based concentration of 150 mg/kg for residents). Lands extending beyond the residential areas (agricultural lands) have been or will be limed and tilled to ameliorate elevated arsenic, copper, and zinc levels, and low soil pH (U.S. EPA 1999). Trigger values for the agricultural areas include an arsenic value of 620 mg/kg (human health risk-based concentration for ranchers and farmers) and the potentially phytotoxic values presented in Table 1.
The Reclamation Research Unit, Montana State University, in conjunction with WBE, Inc. and CH2MHill performed oversight for sampling and remedial actions along the Eastside ditch for the U.S. EPA. Soil samples were collected from 17 residences and adjacent pastures, and 2 pastures in 1998 and Remedial actions were performed by ARCO during the summer and fall of 1999.

5. METHODS
• Residential yards were divided up into discrete sampling units and composite soil samples were collected from 0-2 inch depths. A single test pit was sampled from 0-2 inches, 2-6 inches, inches and inches in each unit. Samples were analyzed for arsenic (As), copper (Cu) and zinc (Zn) by x-ray fluorescence (XRF), and for pH from a saturated paste.
• Agricultural lands were sampled at 0-6 inch and 6-12 inch depths. The sample locations were biased towards impacted areas. The samples were analyzed for As, Cu and Zn by XRF, and pH from a saturated paste.
• Based on the analytical results, response action work plans were developed for each residential yard and pasture area. Residential yards which exceeded 120 mg/kg arsenic were excavated and replaced. The replaced areas were either sodded or re-seeded. Excavated soils were placed in adjacent pastures (agricultural areas). Driveways were limed and tilled if they exceeded the residential trigger value.
• Confirmation samples were collected following excavation to determine remaining As, Cu and Zn levels.
Agricultural areas were limed (< 0.25 mm crushed limestone and prilled calcium oxide), and tilled to depths up to 12 inches using a Rome disc, or up to 24 inches using a Baker plow.

7. RESULTS
• Soil samples were collected from 17 residences and 2 agricultural lands in 1998 and 1999 (Figure 2).
• Soils were excavated and replaced in 8 residential yards as of November 1999 (Figures 3 and 4). Driveway areas were limed and tilled (Figure 5).
• Remaining soil As levels in the excavated areas were typically below 70 mg/kg (Figure 6).
Ten adjacent pasture areas were tilled and limed as of November 1999 (Figure 7).

8. Figure 2. Sample of average As, Cu and Zn values
(0-2 inches) from residential yards before excavation or treatment.

9. Figure 3. Excavation of contaminated soils.

10. Figure 4. Backfilled and sodded residential yard.

11. Figure 5. Liming and tilling driveway.

12. Figure 6. Average arsenic values before and after excavation in some residential yards.

13. Figure 7. Liming and tilling pasture area.

14. CONCLUSIONS
• Excavation and replacement of soils in 8 residential yards has lowered the arsenic typically below 70 mg/kg.
• Tilling and liming of agricultural areas has lowered surface (0-2 inches) arsenic concentrations below the trigger values, and provided for increased soil pH levels above 7.0 standard units. Copper and zinc concentrations have been reduced to levels below the phytotoxic thresholds presented in Table 1.

15. REFERENCES
AERL Draft final phase I post construction report for the
Unilateral Administrative Order for the Deer Lodge Valley Irrigated Lands Time Critical Removal Action. ARCO Environmental Remediation. Anaconda, MT.
AREL Response action construction work plant for select historically irrigated areas, Clark Fork River Operable Unit. ARCO Environmental Remediation, Anaconda, MT.
CDM Federal Programs Area Regional Water, Waste and Soils Operable Unit final baseline ecological risk assessment, October. CDM Federal, Helena, MT.
NRDP Terrestrial resources injury assessment report upper Clark Fork River NPL Site. RCG/Hagler Bailly. January 1995.
U.S. EPA Unilateral Administrative Order for the Deer Lodge Valley Irrigated lands Time Critical Removal Action, EPA Docket No. CERCLA