0071-2858-MTDC: Adit Discharge Summary for the Elkhorn and Charter Oak Mines, MT: The Elkhorn Mine

Some 12 to 14 inches of thick ferric-oxyhydroxide precipitate formed when the stream was channeled, changing the water level in the portal. The discharge ranged from a minimum of about 80 gallons per minute to a maximum of about 130 gallons per minute during the period of record (figure 8). The greatest discharge occurred in May and June during snowmelt and spring storm events and generally declined throughout the rest of the year.

[image] Graph showing peak months of discharge at the Elkhorn Mine.

Figure 8—The flow from the Elkhorn adit does not
peak until June. The flow remains high until November,
demonstrating that ground water from a
large area influences the flow.

Temperature, pH, and specific conductivity show some seasonal trends, particularly in the spring months (figures 9a, 9b and 9c). Water temperature, for example, follows a seasonal trend overall, with the lowest temperature in late winter. The annual range in temperature, however, is less than 2 degrees Celsius—suggesting a deeper ground water flow source. Overall, water quality appears to be best just before the spring melt and poorest during higher flows. The range of values throughout the year, however, are small. The probes were fouled by precipitating ferric oxyhydroxides (a rustlike substance, figure 10), repeatedly degrading the dissolved oxygen and oxidation reduction potential data. Table 2 shows metals in the adit discharge at the Elkhorn Mine.

[image] Graph showing seasonal changes in discharge rates at the Elkhorn Mine.

Figure 9a—Seasonal changes in discharge rates at
the Elkhorn Mine did not coincide with the greatest
fluctuations in temperature (figure 9a), pH (figure 9b),
or specific conductivity (figure 9c).

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Figure 9c—Fluctuations in specific conductivity at
the Elkhorn Mine from July 1998 to June 2000.

[photo] Recorder sonde with ferric hydroxides precipitating on probe.

Figure 10—Ferric hydroxides precipitated on the probes of the recorder sonde
within a few days. The quality of the measurements of some parameters
such as oxidation reduction potential and dissolved oxygen degraded
as the probe was fouled with precipitates.

Table 2—The concentration of metals in the adit discharge remained stable when the flow varied during different seasons at the Elkhorn Mine
Sample Date	pH	Fe (mg/l)	S04 (mg/l)	Al (mg/l)	As (mg/l)	Cd (mg/l)	Cu (mg/l)	Ni (mg/l)	Zn (mg/l)	Pb (mg/l)
9/9/1996	7.9	<0.003	247	<30	<1.00	<2.0	15	7.8	<2.0	28
11/25/1998	6.16	0.036	111	<30	4.2	12.6	264	5.8	<2.0	2,380
6/2/1999	6.37	0.316	105	110	4.01	10.2	381	<2.0	16.3	3,520
7/30/1999	5.52	0.334	103	360	3.44	17.2	526	2.2	80.2	2,860
9/2/1999	5.61	1.49	118	232	3.29	14.9	483	2.4	36.9	2,810
10/5/1999	6.17	0.309	103	126	3.39	13.1	283	2.6	7.8	2,380
11/9/1999	6.06	0.193	103	108	3.36	11.9	263	2.5	9.8	2,390