উন্নত করার কিছু দেখছেন? একটি পরিবর্তন প্রস্তাব করুন।
A ranch downstream of the "Richest Hill on Earth"
Grant-Kohrs Ranch, founded in the 1860s by stockgrower Johnny Grant and cattle baron Conrad Kohrs, began 120 years as one of the largest open-range cattle operations in the American West. It became a national historic landmark in 1960 and a unit of the National Park Service in 1972. In 2023 the national historic site covered 1,618 acres near Deer Lodge, in southwestern Montana, and some 15,000–20,000 visitors a year come to experience the cattle industry's part in Western culture through its original buildings, family furnishings, ranching equipment and working livestock.
It is also the only NPS unit on the National Priority List for Superfund cleanup.
From 1864 to the 1980s, the Butte mining district — once called the "Richest Hill on Earth" — mined gold, silver, copper and lead, and brought great wealth to the region. Its mining and milling wastes were dumped in the upper Clark Fork basin, spreading material rich in cadmium, copper, iron, lead, manganese, zinc and arsenic through the soils, streams and floodplains of the Deer Lodge Valley. Exposed tailings left acidic, metal-laden bare patches called "slickens," damaging habitat on land and in water. Dead vegetation and fish kills blamed on heavy metals led the Environmental Protection Agency in 1989 to declare a 120-mile stretch of the Clark Fork, including the ranch, a Superfund site.
The study area: Grant-Kohrs Ranch National Historic Site, USGS streamgages and Superfund units in the upper Clark Fork basin. USGS
The cleanup
With oversight from the EPA and the Montana Department of Environmental Quality (MDEQ), the NPS began cleaning up the 2.6 miles of the Clark Fork that run through the ranch, where the floodplain and banks held extensive mining wastes.
| Began | November 2018 |
| Planned | excavate and remove 400,000 cubic yards of contaminated soil, then landscape, add topsoil and replant |
| Done | November 2020 — 320,000 cubic yards removed; replanting carried out as clean soil went back in |
Work Area G on the Clark Fork before cleanup, May 2017. USGS
Work Area G: aerial view (May 2017), before cleanup (May 2017) and after (May 2023). Photos A and B courtesy of Jeff Johnson, NPS, Grant-Kohrs Ranch; photo C courtesy of USGS.
Why sample more often
Montana's aquatic-life standards, set by MDEQ, protect fish and other aquatic life by limiting a pollutant's concentration, how long exposure lasts and how often a limit may be exceeded:
| Standard | Exposure | May be exceeded |
|---|---|---|
| Acute | 1 hour | once, on average, in 3 years |
| Chronic | 96 hours | once, on average, in 3 years |
These standards are what MDEQ uses to decide whether a river is impaired and needs rehabilitation.
The USGS has sampled the upper Clark Fork since 1985–88 through a small long-term monitoring network, with the EPA and MDEQ. It samples eight times a year at two streamgages that bracket the ranch — Clark Fork at Deer Lodge (12324200) upstream and Clark Fork above Little Blackfoot River near Garrison (12324400) downstream — for streamflow, metals, nutrients and suspended sediment. Eight samples a year are too few to see what the cleanup was doing to the river as it passed through the ranch.
Turbidity as a stand-in
So in 2019, the USGS and NPS began a 2-year study to estimate metal concentrations in the river continuously during the cleanup.
The idea rests on turbidity — how cloudy the water is. Suspended particles such as sediment, algae and plankton make water murky; more particles mean higher turbidity. Turbidity sensors measure light scattered back from those particles, and the USGS has a strong record of using them to predict suspended-sediment concentrations. Fine sediment — clay and silt particles smaller than 0.063 mm, plus some fine sand and organic matter — carries metals on its surfaces: its particle size, surface area and surface electrical charge let it collect and concentrate them. So a statistical link between turbidity and measured metal concentrations lets turbidity stand in for the metals.
- Turbidity sensors were installed at both streamgages.
- Four extra sampling events a year were added in 2019 and 2020.
- Sensors recorded every 15 minutes and reported by satellite every hour to the USGS office in Helena.
- The turbidity–sediment relation was extended to predict cadmium, copper, iron, lead, manganese, zinc and arsenic, published in near real time on a public USGS web page.
Spikes and drops in the estimates can be matched to cleanup work: excavation, streambank restoration and replanting.
What the sensors showed
Where the copper comes from. During early snowmelt runoff in 2019 and 2020, copper peaked before streamflow did, while flows were still fairly low, at both streamgages. That suggests the copper came from stream-bed sediments and nearby banks and floodplains close to the gauges, rather than from farther up the basin.
How often copper broke the standards:
| Streamgage | Chronic standard exceeded | Acute standard exceeded |
|---|---|---|
| Clark Fork at Deer Lodge | 90% of the time when flow topped 200 ft³/s | 85% of the time above 260 ft³/s |
| Clark Fork near Garrison | 85% of the time above 250 ft³/s | 50% of the time above 280 ft³/s |
What comes next
Water quality may improve as MDEQ's cleanup of contaminated floodplain soils upstream of Deer Lodge proceeds, alongside the work on the ranch. Cleaning up the banks and floodplains between the ranch and Garrison — still being planned — is key to measurable improvement. The hope is that MDEQ can eventually take the Clark Fork off the EPA's 303(d) list of impaired waters. The study's data are published in a USGS data release; water data are in the USGS National Water Information System.
Sources
Based on Ellison C, "Predicting water quality in the Clark Fork near Grant-Kohrs Ranch National Historic Site, southwestern Montana," U.S. Geological Survey Fact Sheet 2023-3032; a work of the United States government in the public domain.
লাইসেন্স: CC0 1.0 (পাবলিক ডোমেইন) · গৃহীত হয়েছে pubs.usgs.gov
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