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Draft Final Report Acwa-08-05 - Continuation Of Lower Nushagak River Fecal Coliform And Water Quality Assessment - Alaska Department Of Environmental Conservation - 2008 Page 10

DRAFT Final Report ACWA-08-05

8 October 2008

to around 20µS/cm during the May sampling event, when the river stage was at its highest during our

five sampling events. As the river level dropped in June, the specific conductance came back up to near

50µS/cm.

Turbidity

Turbidity is a measure of water’s cloudiness due to suspended particles that are generally difficult to see

with the unaided eye. Turbidity tends to increase during high flow conditions, due to the runoff from

rainfall or snowmelt carrying particles from the land into the river. Turbidity levels were generally low in

the months of August, September, and October, with average turbidity measurements of 1.9, 2.0 and

4.1 NTUs respectively. During the May sampling event, the average turbidity measurement was 25.0

NTUs, a six-fold increase over the readings in October. As the river level dropped in June, so did the

turbidity, with an average measurement of 4.8 NTUs. The Alaska state water quality standard for

turbidity requires natural running waters not exceed an increase of 25 NTUs above natural conditions.

Fecal coliform bacteria

Fecal coliform bacteria are used as indicators of possible sewage contamination, as they are commonly

found in human and other animal feces. The Alaska state water quality standards for fecal coliform

(drinking water) state that the geometric mean for a 30-day period should not exceed 20 fecal

colonies/100mL, with no more than 10% of the samples exceeding 40 fecal colonies/100mL. At least

five samples should be collected over the 30-day period for the calculation of the geometric mean; our

study design didn’t allow for such a calculation.

The May 2008 sampling event showed a spike in fecal coliform concentrations throughout the lower

Nushagak River. Fecal coliform bacteria concentrations ranged from a low of 36 fc/100ml near

Koliganek to 91 fc/100ml above Koliganek near the opposite bank. River levels during that sampling

event were relatively high (Figure 3). River levels were also relatively high during the October 2007

sampling event, yet fecal coliform concentrations were lowest throughout the river at that time, ranging

from a low of 1 fc/100ml near the Mulchatna River mouth to a high of 18 fc/100ml along the affected

bank downstream from the Mulchatna River mouth.

Figure 3.

River Stage (observational) data for the Nushagak River at Ekwok.

Page 10

Draft Final Report Acwa-08-05 - Continuation Of Lower Nushagak River Fecal Coliform And Water Quality Assessment - Alaska Department Of Environmental Conservation - 2008 Page 10

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