Studies of Suction Dredge Gold-Placer Mining Operations Along 


the Fortymile River, Eastern Alaska

USGS Fact Sheet FS–154–97


October 1997

U.S. Department of the Interior


U.S. Geological Survey

The U.S. Geological Survey (USGS) 

and the Alaska Department of Natural 

Resources (AKDNR) are investigating 

the environmental geochemistry of the 

Fortymile River drainage area in 

eastern Alaska. This river is designated 

a Wild and Scenic Corridor by the 

Alaska National Interest Lands 

Conservation Act. Current users of the 

river include placer mine operators, as 

well as boaters and rafters. Along the 

North Fork Fortymile River, and just 

below its confluence with the South 

Fork (fig. 1), mining is limited to a few 

small suction dredges which, combined, produce as much as a few 

hundred ounces of gold per year. In 

this area, some potential environmental concerns have been raised 

associated with the mining activities, 

including increased turbidity of the 

river water; adverse impact on the 

overall chemical quality of the river 

water; and potential additions of 

specific toxic elements, such as 

arsenic, to the river during mining 

operations.


 A cooperative effort between the 

USGS and the AKDNR was initiated 

to provide data to address these concerns as well as regional geochemical 

data. In June 1997, field measurements 

were made for pH, turbidity, electrical 

conductivity (a measure of the total 

dissolved concentrations of mineral 

salts), and stream discharge for the 

Fortymile River and many of its tributaries. Samples were collected at the 

same time for chemical analyses, 

including trace-metal analyses, in the 

USGS laboratories in Denver, 

Colorado. This Fact Sheet summarizes 

some of the results of this ongoing 

study, especially in regard to the 

suction dredge mine sites.

TURBIDITY SURVEYS 

Two sites were studied 

where suction dredges 

were operating in the 

North Fork Fortymile 

River, as shown on 

figure 1. Samples were 

collected on a grid 

extending downstream 

from the dredges as 

they were operating 

and compared to 

measurements made 

upstream of the 

dredges. One dredge 

had a 10-inch diameter 

intake hose and was 

working relatively fine sediments on a smooth but fast section of the river. The 

other dredge had an 8-inch intake and was working coarser sediments in a 

shallower reach of the river. State regulations require that suction dredges may not 

increase the turbidity of the river by more than 5 nephelometric turbidity units 

(NTU), 500 feet ( 150 m) downstream. In both cases, the dredges were well 

within compliance with this regulation. The results of the turbidity survey for the 

10-inch dredge are shown on figure 2. Turbidity values behind the 8-inch dredge 

were lower, because the smaller intake was moving less sediment material, and 

because the coarser sediments being worked by the 8-inch dredge settled more 

rapidly.

20 feet100 feet

1.7 1.5

1.7

1.5

2.1

1.3 1.4 1.6 2.2 1.8 1.6

1.6 1.4 1.5 2.2 2.2

1.4 1.4 1.8 2.3

1.3 2.0 2.0 2.1 3.7

1.5 19 1.9 1.6

Left Bank

dredge

Figure 2. Results of turbidity 

survey behind an operating 

10-inch suction dredge (site 

#1 on fig. 1). All numbers 

shown are in NTU, or 

nephelometric turbidity units; 

the standard unit of turbidity. 

The right bank of the river is off 

the edge of the figure. The 

approximate shape of the 

plume is shown in gray. Note 

that the figure is exaggerated 

5x horizontally, so the plume is 

actually much narrower than 

it appears in the figure. To 

comply with State regulations, 

dredges may not increase the 

turbidity of the river by more 

than 5 NTU, 500 feet behind 

the dredge.

North Fork

Fortymile River

Fortymile River

scale 1:500,000 (1 inch equals approx. 8 miles or 12.8 km)

oS u

ht Fork

Sample location

Actively mined area

EXPLANATION

Figure 1. Schematic map of the Fortymile River study area. 

The town of Chicken lies about 20 miles (32 km) to the south.

Operating dredges #1 and 2–


sites of more intensive


chemical studies

Uhler Cr.

 Alaska

Location of study area


in eastern Alaska

#1 #2
0 5 10 15 20 25

0

5

10

15

20

25

RANGE OF TURBIDITY, NTU

total number of samples = 73

NUMBER OF SAMPLES

samples from dredge areas

samples from unmined areas

Dredge 1 Dredge 2

1A 1B 1C 2A 2B 2C

pH 7.7 7.6 7.8 7.0 7.5 7.5


Arsenic	 0.3 0.3 0.3 0.3 0.3 0.3


Iron 110. 110. 110. 100 97 100


Chromium 2 2 3 3 3 3


Cadmium all less than 0.02 micrograms per liter


Cobalt 0.07 0.07 0.06 0.06 0.05 0.05


Zinc 	 0.8 0.6 0.8 1.0 1.0 1.0 


Lead all less than 0.05 micrograms per liter


Figure 3. A comparison of 

turbidity values between 

mined and unmined areas 

shows that the suction 

dredge mining does not 

affect the turbidity of the 

Fortymile River system 

under the conditions 

studied. The highest 

turbidity values from the 

dredge areas are within 

200 feet (60 m) of the back 

of the two operating 

dredges which were 

studied.

Comparison of Dredge Turbidity 

to Regional Values

 The turbidity values found in the 

dredge studies fall within the range of 

turbidity values found for currently 

mined areas of the Fortymile River and 

many of its unmined tributaries. 

Figure 3 shows the ranges of turbidity 

values observed along the horizontal 

axis, and the number of samples which 

fall within each of those ranges. For 

example, 25 samples had turbidity 

between 1.0 and 1.5 NTU, 22 of which 

were in a dredged area. The highest 

turbidity value was from an unmined 

tributary to Uhler Creek; the lowest 

from a number of different tributaries 

to the North Fork. As seen on the 

figure, there is no appreciable 

difference in the distribution of 

turbidity values between mined and 

unmined areas.


CHEMICAL SURVEYS

 Water-quality samples were 

collected at three points 200 feet 

behind each of the two operating 

suction dredges. One sample was 

collected on either side of the plume, 

and one in the center of the plume. 

The samples were passed through a 

filter with a nominal pore size of 0.45 

micrometers and acidified to a pH less 

than about 2. Results are shown in the 

table below. Samples 1A, 1C, 2A, and 

2C are from either side of the plume 

behind dredges 1 and 2, respectively. 

Samples 1B and 2B are from the 

center of each plume. All concentrations given are in micrograms per liter, 

except pH, which is expressed in 

standard units.

 The data show similar waterquality values for samples collected 

within and on either side of the dredge 

plumes. Further, the values shown in 

the table are roughly equal to or lower 

than the regional average concentrations for each dissolved metal, based 

on the analyses of 25 samples collected throughout the area. Therefore, 

suction dredging appears to have no 

measurable effect on the chemistry of 

the Fortymile River within this study 

area. We have observed greater variations in the natural stream chemistry in 

the region than in the dredge areas. 

Preliminary results from three undisturbed sites suggest that the shallow 

ground water may be of poorer quality 

in some pyrite-rich outcrop areas. 

This water could be discharging to the 

surface water, leading to increased 

dissolved metal loads.


CONCLUSIONS

 The data collected for this study 

help establish regional background 

geochemical values for the waters in 

the Fortymile River system. As seen in 

the chemical and turbidity data any

variations in water quality due to the 

suction dredging activity fall within 

the natural variations in water quality. 

This conclusion is further supported by 

the other water-quality data collected 

throughout the region, at the sites 

shown on figure 1.


 Current and future work on the 

Fortymile River system is directed at 

understanding the natural variations in 

water chemistry throughout the area. 

The primary factors causing these 

variations are probably variations in 

bedrock geology and mineral content, 

as the surface waters come into contact 

with the bedrock throughout their flow 

history.


REFERENCES TO CONSULT

The USGS website: http://www.usgs.gov

Gough, L., Day, W., Crock, J., Gamble, B.,


 and Henning, M., 1997, Placer-Gold


 Mining in Alaska–– Cooperative studies


 on the effect of suction dredge opera- 


 tions on the Fortymile River: U.S. Geo-


 logical Survey Fact Sheet FS-155-97.


FOR MORE INFORMATION, 

CONTACT

Richard B. Wanty


U.S. Geological Survey


M.S. 973, Denver Federal Center


Denver, CO 80225


Bronwen Wang


U.S. Geological Survey


4230 University Drive


Anchorage, AK 99508-4664


Jim Vohden


Alaska Department of Natural Resources

3700 Airport Way

Fairbanks, AK 99709