OCR Text |
Show low ( figure B), there was greatly reduced south- to- north flow, and the north- arm salinity was increasing due to evaporation. Thus, the overall 1995 to 1998 path is still below that of the 1966- 1987 path. For both the south and north arms ( figures G and H) the change in wt% salt per foot of elevation change is shown adjacent to a dashed line which approximates the slope of the different paths within the 4198 to 4202 elevation range. For the south arm ( figure G), curve " a", representing the 1966- 1987 path of the lake, changes at the rate of about 0.77 wt%/ foot of elevation change. Curves Mbw and Mc", with values of 1.22 wt%/ ft., and 1.2 wt%/ ft., respectively, both show a greater change in the lake's wt%/ ft of elevation change. This increase is due to the south- arms increase in overall salinity brought about by the large influx of dense, north- arm brine when the breach was opened in 1984. For the north arm ( figure H), curve " a" represents the 1966- 1987 path of the lake, and changes at the rate of about 0.47 wt%/ ft of elevation change. Curve " b", with a value of 0.93 wt%/ ft of elevation change, represents the lake between 1987 and 1995, as it was dropping rapidly, and being diluted by south- arm brines after the causeway was breached. It also represents the loss of high- salinity brines to the south through return flow through the end of 1998. Curve " c", with a value of 0.38 wt%/ ft of elevation change, represents the lake's path from 1995 into 1999. This time period represents a gain of salt through nearly- exclusive south- to- north flow of south- arm brine through the breach opening until 1999 when north- to- south return flow was again observed. South- Arm Deep Brine Layer Stratified- brine conditions in GS: ( where a low- density brine overlies a brine of greater density) were first observed in the south arm during the early 1960s ( Hani and Handy, 1969, and UGS brine data files). These conditions were observed until mid- 1991, near the end of the high- water period, at which time the lake became vertically mixed. During 1999, the lake appears to be showing signs of restratification. Published ( Hani and Handy, 1969, Mason, 1970, Spencer and others, 1985, and Whelan, 1973) discuss a possibly significant difference in the chemistries of the pre- 1991 north arm of GSL, and the deep- brine layer in the south arm. The brines in the main southern body of the south arm and in the northwest portion of the south arm are separated by a low topographic ridge on the lake's bottom between Promontory Point and Carrington Island. Tri- linear plots using chemical data from Hani and Handy ( 1969), the Utah Department of Health, and the UGS were used to evaluate and compare the chemistries of the north- arm brines, and brines from the two areas in the south arm. Brines analyses from the 1960s suggest that there was a change from lower sodium- chloride north- arm brines to higher sodium- chloride main- body south arm brines ( as end members) as precipitated salt was dissolved. Brines in the northwest portion of the south arm had an intermediate sodium- chloride content ( on a dry- weight- percent basis). Tri- linear plots of more recent brines ( 1978 to 1984 Department of Health data, and pre- 1991 UGS data) from the 376 |