WSP07478 - Laserfiche WebLink

Home Browse Search

tv o -.J ..... ~mhos/cm @ 250C, The conductivity probe was buried under sediment, and a delayed response masked the shape and timing of the halograph. While some sediment induced error is probable, the above maximum and minimum conductivity values are close. Cosl Creek flow and Quality measurements Conductivity and flow measurements made on Coal Creek during 1976 are plotted on Figures 4.8 to 4.15 inclusive for sites shown on Figures 3.1 and 3.2. The average observed streamflow in Coal Creek, below the Book Cliffs, declined from 1.5 cfs in April to 0.25 cfs in August (Figure 4.9). The mix of anions and cations at the upper site was fairly constant (Appendix C, Table C-2). Conductivity increased from an average of 750 ~mhos/cm at 250C in April to 1000 ~mhoB/cm at 250C in October with measurements made every 30 days (Figure 4.8). Sharply lower values of conductivity were observed after a storm event. This is attributed to the dilution effects of overland flow and to the low quantities of residual salta held in the sediments of the Coal Creek channel. Linear regression analyses were applied to estimate six chemical constituents using conductivity as the independent variable. The t-test was used to test the null hypothesis that the slope of the regression line equals zero. I " y ~ a + b [Conductivity] . . . . . (4.1) in which Y TDS or individual ion concen- tration a and b = Constants , The results are shown in Table 4.1. The low correlation coefficients were due primarily to grouping of the observed values within a very small range; this is particularly evident at the spring where the quantities of flow and chemical constituents varied in too small a range for meaningful regression to be possible. f At no time were overland return flows from the irrigated land associated with an increase in conductivity of more than 10 percent of that measured at the upper site. Because of seepage, the flow diminished and often disappeared in the 3-mile section below the upper site (Figure 3.1), Approximately 3 miles below the Book Cliffs, water enters Coal Creek from numerous sIDall seeps and one, large spring. The source and the extent of the aquifer supplying the seeps and spring are unknown (Gwynn 1976). Discharge and water quality at the spring were monitored. Flow (Figure 4.11) was observed to peak at 0.1 cfs during April and to steadily decline to 0.04 ds during December. Conductivity (Figure 4.10) remained I ~ t I I I l stable with an observed mean of 2759 ~mhos/cm at 25'C and a standard deviation of 235 ~mhOS/CID at 250C. Data presented in Appendix C (Table C-2) also show that the concentraC tions of the chemical ions in the spring discharge were nearly constant. The middle sampling site was located approximately 3.25 miles below Coal Creek's emergence from the Book Cliffs (Figure 3.1). The observed flows were generally low, except following storm events, and came from the spring and seeps immediately upstream (Figure 4.13). The conductivity ranged from approxi- mately 1000 ~mhos/cm at 250C to 3200 umhos/cm at 250C (Figure 4.12). The large variation in conductivity was due to dilution by storm runoff. At low flows, the majority of the flow originated as groundwater of approxi- mately 2760 umhos/cm at 250C. At high flowa the majority of the flow originated as sur- face runoff from either the upper part of the subbasin or above the upper site and ex- hibited little channel salt uptake. Parti- cularly high correlations with conductivity (Table 4.1) were obtained at this site for TDS and sulfate. The flow at the lower site, 8.2 miles downstream from the Book Cliffs (Figure 3.1), was highly ephemeral (Figure 4.15). Much of the flow passing the middle site was lost through channel seepage and evaporation between the two sites. During periods of continuous flow, very little salt uptake occurred in the Coal Creek channel, and the conductivity of the lower site approached that of the upper site (Figure 4.14). During periods of low flow, when groundwater repre- s ented the major source of flow, the con- ductivity equaled or exceeded the mean groundwater conductivity. From Table 4.1 high correlati.m coefficients (Equation 4.3) were obtained for TDS, sulfate, magnesium, and chloride, The null hypothesis waS rejected at the 0.99 confidence level for all seven regressions. Mean measured values of anions and cations at each site are listed in Table 4.2. On a given date, TDS measurements at the middle and lower sites usually were very close (Appendix C, Table C.2). The smaller mean value of the TDS at the lower site (Table 4.2) is explained on the basis that a larger number of samples were taken at this loca t ion than a t the middle site dur ing spring runoff. Salinity from the Coal Creek channel sediments The natural channel bottoms in the Coal Creek basin are composed of unconsolidated bed material and exposed Mancos Shale. The channels display surface efflorescence varying from a dense white blanket to inter- mittent small discrete deposits. Mass transport of the channel bed material by major storm events was observed during the study reported here and by Mundorff (1972). During relatively steady and uniform low flow 29