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Kansas Geological Survey, Open-file Report 88-39
Great Plains and Cedar Hills Aquifers--Page 14 of 25


6.3.2 Identification of Salinity Sources Using Mixing Curves

The origin of the present salinity of waters in the Great Plains aquifer was determined from mixing curves on plots of Br/Cl, I/Cl, and B/Cl versus chloride concentration based on the method of Whittemore (1984, 1988). Points for ground waters collected during this study fit within boundaries for the mixing of fresh waters with saltwaters containing Br/Cl and I/Cl weight ratios appreciably below those for oil-field brines from the study area (Figures 29 and 30). Weight ratios of B/Cl for oil-field brines range from the same as to only slightly higher than the B/Cl of the expected saltwater source for the mixing zone enclosing points for the aquifer waters (Figure 31).

Figure 29. Weight ratio of Br/Cl x 104 versus chloride concentration for KGS data. Well waters with nitrate concentrations <10 mg/L (+) and >10 mg/L (X); multiple-completion monitoring wells (*); oil-field brines (O); seawater (small filled circle).

Although the B/Cl and I/Cl ratios for seawater fall within the expected range for the salinity source for the Great Plains aquifer waters, connate water similar to the composition of seawater cannot be the major source of salinity because the Br/Cl ratio for seawater is much higher than for the expected saltwater source. Points for waters sampled from the Haberer No. 1 well in Russell County (NW, SW, NW sec. 23, T12S, R15W) drilled by the U.S. Geological Survey are also plotted on Figures 29 and 30, and labeled HD for water from the Dakota Formation and HC for water from the Cheyenne Sandstone and Cedar Hills Sandstone.

Figure 30. Weight ratio of I/Cl x 106 versus chloride concentration for KGS data. Well waters with nitrate concentrations <10 mg/L (+) and >10 mg/L (X); multiple-completion monitoring wells (*); oil-field brines (O); seawater (small filled circle).

The Haberer well samples plot near the center of the saltwater end of the mixing zones for Br/Cl and I/Cl drawn for the samples collected from the present study. The range in the Br/Cl, I/Cl, and B/Cl ratios for the expected saltwater source are all somewhat higher than the ratios for halite-solution brines from the Cedar Hills Sandstone and Salt Plain Formation in eastern Stafford County (Whittemore et al., 1987). The ranges of Br/Cl and B/Cl ratios for the saltwaters from the Permian rocks in Stafford County are 1-2 x 104 and 9-24 x 105. The Br/Cl in the Permian saltwaters in Stafford County is about the same as for that in halite-solution brines from the Hutchinson Salt Member of the Wellington Formation, while the B/Cl is higher than the 2.5-7 x 105 range for the Wellington (Whittemore, et al., 1981). Although only a few iodine determinations are available for saltwaters from the Permian bedrock in Stafford County, the values for I/Cl appear to be in the same range as for halite-solution brines from the Wellington Formation, 1.4-3 x 106.

Figure 31. Weight ratio of B/Cl x 105 versus chloride concentration for KGS data. Well waters with nitrate concentrations <10 mg/L (+) and >10 mg/L (X); multiple-completion monitoring wells (*); oil-field brines (O); seawater (small filled circle).

The major salinity source for ground waters from the Great Plains aquifer sampled during the current study thus appears to be halite- solution derived from underlying Permian rocks. The higher ratios of Br/Cl for saltwaters in the Permian rocks of Stafford County and in the Wellington Formation farther to the east might reflect small amounts of waters similar in chemistry to seawater remaining in the mixture after the intrusion of the Permian saltwaters. The slightly higher I/Cl and B/Cl for the expected salinity source for the Great Plains aquifer could be the result of chemical changes occurring during the flow of water through clays. Boron concentrations could have increased during ion exchange as described above, and small amounts of iodine could have been leached from higher contents of organic matter that are often associated with clays, particularly marine clays.

Only one well-water sample collected from the Cretaceous rocks during the areal survey of water quality may have been affected by a saltwater source with a Br/Cl similar to oil-field brine. The sample site (RH 12) is in northern Rush County. If the water is a mixture of ground water having a chemistry that would place it below the mixing curve boundary for freshwater and halite-solution brine in Figure 29, with oil-field brine in the middle of the Br/Cl range for oil brine in this figure, the chloride contribution from the oil brine would need to be at least 200 mg/l. No oil fields appear on the Kansas Geological Survey map of oil and gas in Kansas in the area of this well, however. Additional investigation of this site is needed to explain the anomalous chemistry of the ground water.

The other two points for waters collected in this study that fall within the freshwater and oil-brine mixing zone are for the Gorham monitoring site. The results of the identification are discussed under the section on the monitoring sites.

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Kansas Geological Survey, Dakota Project
Original document dated December, 1988
Electronic version placed online April 1996
Scientific comments to P. Allen Macfarlane
Web comments to webadmin@kgs.ku.edu
URL=http://www.kgs.ku.edu/Dakota/vol3/KCC/kcc14.htm