Karst aquifers typically have desirable physicochemical properties and can provide an important source of groundwater for drinking. Kiyan karst springs, located in the geological region of the Zagros ...(western Iran), consist of two springs (Keder and Zolal) situated 3 m apart. The water of Zolal spring is always clear, with no turbidity during rainfall or earthquake phenomena, but Keder spring, which also provides drinking water to a city of 10,000 inhabitants (Kiyan city), has very high turbidity in these conditions. This situation disrupts the drinking water supply for about 1 month per year in the study area. The interpretation of discharge time series diagrams, precipitation, and turbidity in different rainfall and earthquake conditions, composite diagrams, and stable isotope concentrations show this study area to be a well-developed karst environment with a highly eroded tectonic zone and large karst conduits along with turbulent water flow. In general, the difference in the behavior of the two springs is related to different paths of water movement through the drainage area. The two springs share the same source of recharge from rainfall and the same initial underground drainage path from the largest sinkhole in the region; although the path of water movement is divided into two branches, with Zolal spring water entering a narrower fault channel. In the Zolal spring pathway, a siphon-shaped reservoir is formed where muddy rainwater settles, so that the water from this spring is clear but has higher concentrations of some ions analyzed here than Keder spring water.
The aim of this study was to determine by static bioassay whether water hardness affects the toxicity of Zn and Cu to a fish, Gambusia holbrooki Girard, 1859. The acute toxicity of selected heavy ...metals to G. holbrooki was determined in soft, hard and very hard water (25, 125 and 350 mg L
−1
as CaCO
3
). Results showed that water hardness had a significant effect on Cu and on Zn toxicity on fish. Toxicity of Cu and of Zn increased with decreasing water hardness. The 96 hours LC
50
values for G. holbrooki were higher in the hard and very hard water compared with soft water. Water hardness had a much smaller effect upon the acute toxicity of Cu than that of Zn. It was observed that the 96 hours LC50 for Cu at the soft, hard and very hard water was found to be 0.017, 0.17 and 0.65 mg L
−1
, respectively, while the 96 hours LC
50
for Zn at the soft, hard and very hard water was found to be 0.46, 48.1 and 121.6 mg L
−1
, respectively.
