High Water Hardness Research Articles

Effects of Water pH and Hardness on Infection of Silver Catfish, Rhamdia quelen, Fingerlings by Ichthyophthirius multifiliis

The aim of this study is to determine the effect of (1) different pH levels (5, 6, 7, 8, and 9) with water hardness 20 mg/L CaCO3 and (2) pH 5 and 7 with varying water hardnesses (20, 60, and 120 mg/L CaCO3) on the intensity of infection by Ichthyophthirius multifiliis in fingerlings of the silver catfish, Rhamdia quelen. In Experiment 1, the lowest and highest mortality rates were observed in fingerlings exposed to pH 5 and 9, respectively; in Experiment 2, fish kept at pH 5 and 7 with hardness 20 mg/L CaCO3 showed significantly lowest and highest cumulative mortality, respectively, than those maintained in other conditions. The lowest intensity of trophonts/fingerling was observed at pH 5 and hardness 20 mg/L CaCO3 in both experiments. We conclude that infection by I. multifiliis is less severe in silver catfish maintained at pH 5 and hardness 20 mg/L CaCO3. High water hardness led to an increased intensity of trophonts and decreased survival in silver catfish kept at pH 5, but decreased intensity of trophonts and improved survival when the fish were kept at pH 7.

Influence of water hardness on concrete surface deterioration caused by nitrifying biofilms in wastewater treatment plants

In aeration tanks of wastewater treatment plants (WWTP), biodeterioration can lead to a partial dissolution and ensuing erosion of the concrete due to an acid attack caused by the nitrifying biofilm covering the tank surface. In this study, concrete samples were exposed in four different WWTP. The CaO content of the cement used, the water-to-cement ratio, and curing affect the resistance of the concrete to acid attack. However, both experimental data and thermodynamic modeling show that water hardness is the dominating parameter for surface erosion. In the presence of high water hardness, the protons produced by the nitrifiers are buffered and the calcite precipitation occurring in all concrete samples is increased. This results in a decrease of porosity near the surface and higher mass transfer resistance against carbon dioxide and therefore in lower deterioration rates of the concrete.

Phosphorus recovery by struvite crystallization in WWTPs: Influence of the sludge treatment line operation

Phosphorus recovery by struvite (MgNH(4)PO(4).6H(2)O) crystallization is one of the most widely recommended technologies for treating sludge digester liquors especially in wastewater treatments plants (WWTP) with enhanced biological phosphorus removal (EBPR). In this paper, phosphorus recovery by struvite crystallization is assessed using the rejected liquors resulting from four different operational strategies of the sludge treatment line. Phosphorus precipitation and recovery efficiencies of between 80-90% and 70-85%, respectively, were achieved in the four experiments. The precipitates formed were mainly struvite, followed by amorphous calcium phosphate and, in some experiments, by calcite. The highest global phosphorus recovery taking into account both the sludge line and the crystallizer was achieved when mixed thickening and high elutriation were carried out (8.4 gP/kg treated sludge). However, low struvite content was obtained in the crystallizer with this operation scheme due to the high calcium content in the elutriation stream. Therefore, if the final purpose is to obtain struvite, the reduction of the elutriation flowrate is widely recommended in the case of high water hardness.

Natural organic matter (NOM) removal in a typical North-China water plant by enhanced coagulation: Targets and techniques

Seasonal coagulation objectives for a typical North-China water treatment plant with micro-polluted and high alkalinity source waters are proposed in this paper. These are based on a yearlong data collection period where raw water characteristics, trihalomethanes formation potential (THMFP) and coagulation features were investigated using a jar test procedure, resin absorption and ultrafiltration fractionations. Three approaches beyond simply increasing the coagulant dose were used to achieve optimized coagulation. One is coagulation by adjusting the pH of the raw water when metal salt coagulants (FeCl 3 and AlCl 3) are used. At pH levels of about 5.0 for FeCl 3 and about 5.8 for AlCl 3, the highest removal of natural organic matter (NOM) was obtained, which is twice that without pH control. The second is enhanced coagulation through coagulant optimization based on raw water characteristics. A high efficient composite polyaluminum chloride (HPAC) coagulant was developed for the water taking the advantages of polyaluminum chloride (PACl) and other additives. HPAC exhibited 30% more efficiency than AlCl 3, FeCl 3 and polyaluminum chloride (PACl) in dissolved organic carbon (DOC) removal and was also very effective in turbidity removal. The third is enhanced softening with coagulant addition. Higher removal of NOM is achieved by enhanced softening with coagulant addition conditions as compared with conventional coagulation at natural pH. Especially with PACl addition, it can enhance the formation of Mg(OH) 2 precipitate and remove NOM efficiently at a relatively lower pH range (pH < 10). By this approach, the pH for enhanced softening can be decreased significantly into a practical operation pH range for high-hardness water at a treatment plant.

Iron oxidation in different types of groundwater of Western Siberia

The groundwaters of Western Siberia contain high concentrations of iron, manganese, silicon, ammonium, and, in several cases, hydrogen sulfide, carbonic acids, and dissolved organic substances. Generally, the groundwaters of Western Siberia can be divided into two major types: one type with a relatively low concentration of humic substances and high hardness (water of A type) and a second type with a relatively low hardness and high concentration of humic substances (water of B type). For drinking water production, the waters of A type are mostly treated in the classical way by aeration followed by sand bed filtration. The waters of B type often show problems when treated for iron removal. A part of iron practically does not form the flocs or particles suitable for filtration or sedimentation. The aim of this work was to determine the oxidizability of Fe(II), to characterize the iron colloids, and to investigate the complexation of the iron ions with humic substances and the coagulation of the iron colloids in the presence of dissolved organic matter. Water samples of the A and B types were taken from bore holes in Western Siberia (A type: in Tomsk and Tomsk region, B type: in Beliy Yar and Kargasok). Depth of sampling was about 200 m below surface. The oxidation of the groundwater samples by air oxygen and ozone was done in a bubble reactor consisting of a glass cylinder with a gas-inlet tube. To produce ozone, a compact ozone generator developed by Tomsk Polytechnic University was used. For the characterization of the colloids in the water of B type, the particle size distribution and the zeta potential were measured. To investigate the formation of complexes between iron and humic substances in the water of B type, size exclusion chromatography was used. The coagulation behavior of iron in the presence of dissolved organic substances was investigated at different pH values. The agglomerates were detected by measuring the optical density using a UV-Vis spectrometer. Ozone showed, as expected, a faster oxidation of Fe(II) than air oxygen. The rate constants for Fe(II) oxidation were not much different for the waters of A and B types when the same oxidation process was used. However, the removal of iron after oxidation and filtration was higher in the water of A type than in the water of B type. No evidence for the formation of soluble complexes between iron and humic substances were found. In the water of A type, the coagulation process started at pH = 4.5 and accelerated with increasing pH value. In the water of B type, the coagulation of colloids occurred only at pH = 11 and higher. The oxidation experiments indicated no major effect of dissolved organic carbon concentration on the kinetics of Fe(II) oxidation. In contrast to this, the humic substances showed a significant influence on the aggregation behavior of the iron hydroxide colloids. Due to the sorption of humic substances on the iron hydroxide colloids, they were highly stable in the pH range between 4.5 and 10. The particle size measurements confirmed the presence of small colloids in the water of B type. In contrast to this, the iron hydroxide colloids aggregated rapidly at pH = 11. The results showed a great influence of humic substances on the iron removal from groundwaters of Western Siberia with high organic content. The sorption of humic substances on the iron colloids does not obviously allow their coagulation and formation of flocs suitable for filtration or sedimentation. By treatment of groundwaters containing high amounts of humic substances, some problems with the removal of iron are likely to occur. To increase the effectiveness of iron removal, the surface coating and pH-dependent charge effects should be taken into account by the selection and optimization of water treatment processes. The iron colloids coated by humic substances should be separated from the water phase by membrane filtration or by flocculation followed by filtration through different solid materials.

Hard drinking water does not protect against cardiovascular disease: new evidence from the British Regional Heart Study

It has been previously suggested that hard drinking water in general, and in particular high calcium and magnesium intake from drinking water, protect against cardiovascular disease. Prospective study of men from 24 British towns, with widely differing levels of hardness in drinking water. A total of 7,735 men aged 40-59 years were recruited during 1978-1980. Estimates of town-level water hardness were available and tap water samples, taken from 947 participants who also answered a questionnaire about water consumption, were used to calculate individual calcium and magnesium intakes. Men were followed for incident of major coronary heart disease (CHD) and stroke, and CHD mortality for 25 years. Water hardness varied from 0.27 to 5.28 mmol/l in the 24 towns. A weak inverse association was found between water hardness and incidence of cardiovascular disease (CVD) [hazard ratio (HR), 0.96 per two-fold increase, 95% confidence interval (CI), 0.91-1.01, P=0.08 after adjustment for age and seven established coronary risk factors]. No association was observed with CHD incidence (adjusted HR, 0.99, 95% CI, 0.94-1.04, P=0.62) or mortality (adjusted HR, 0.96, 95% CI, 0.90-1.02, P=0.18). Individual magnesium intake showed a positive, rather than an inverse, association with CHD incidence (adjusted HR, 1.10 per two-fold increase, 95% CI, 1.01-1.20, P=0.045); individual calcium intake was unrelated to CHD or CVD end points. This study suggests that neither high water hardness, nor high calcium or magnesium intake appreciably protect against CHD or CVD. Initiatives to add calcium and magnesium to desalinated water cannot be justified by these findings.

Increased Water Hardness and Magnesium Levels May Increase Occurrence of Urolithiasis in Cows from the Burdur Region (Turkey)

Objectives of the study were to measure water hardness in Burdur, and to establish its possible association with urolithiasis in cattle. Water samples were obtained from different stables (n = 15). Water hardness and the concentrations of potassium, calcium, magnesium, sodium, iron, zinc, manganese and copper ions were calculated from these water samples. Total hardness of the samples (mean 285 ppm) exceeded the standards and the water was characterized by high content of magnesium ions. Kidneys (n = 500) were collected randomly from slaughterhouses and examined for urolithiasis. Urolithiasis was observed in 102 kidneys (20.4%). The weights of the stones were between 0.02 and 237.44 g and the colour varied from white to brown. The calculi collected had various shapes and composed of calcium apatite (42.45%), struvite (20.15%), magnesium carbonate (15.15%), calcium carbonate (12.12%), and calcium phosphate cystine (10.13%). It was concluded that high water hardness with high magnesium ion concentrations in water may contribute to urolithiasis and needs to be investigated further in future studies.

Impact of water quality on setting of irreversible hydrocolloid impression materials

Setting of irreversible hydrocolloid impression materials is based on the ionic reaction between carboxylic groups and calcium ions and may, therefore, be affected by ionic species present in the mixing water. The impact of this phenomenon on the clinical performance of these materials has not been well documented. The purpose of this study was to compare the setting behavior of irreversible hydrocolloid impression materials when mixed with tap and distilled water, and to determine the impact of typical cations present in tap water and their concentrations on the setting process. Six brands of irreversible hydrocolloid impression materials (Kromopan 100, Xantalgin Select FS, Alginoplast, Elastic Plus, Ypeen, and Ypeen Premium) were mixed with tap and distilled water (control) according to manufacturers' recommendations. Elastic Plus was also mixed with aqueous solutions containing various concentrations of NaCl, CaCl(2), and AlCl(3) to determine the role of typical cations on setting. Using a controlled shear stress oscillatory rheometer, time changes of storage (G') and loss (G'') moduli during setting were measured at 23 degrees C and used to determine the working and setting times and rigidity of set impression materials. The sample size (n=3) for each material/mixing system was increased to 8 to increase reliability of measurements in systems where the effect of mixing water was low or variance of results was high. The data were analyzed (alpha=.05) using a t test (tap water), a 1-way ANOVA, a Tukey post hoc test (shear stress), and a nested ANOVA and Fisher Least Significant Difference post hoc analysis (cation and cation concentration). Statistical analysis showed significant (P<.001) acceleration in the setting rate for Kromopan 100, the working time of which was shorter with tap water by 23.4 seconds and the setting time, by 32.8 seconds. Similar significant reductions (in seconds) in both working and setting times, respectively, were found with Xantalgin Select FS (12.0, P<.001 and 23.6, P<.002), Alginoplast (24.3, P<.001 and 44.7, P<.005), and Elastic Plus (23.0, P<.001 and 22.0, P<.002), when compared with the same materials mixed with distilled water. At the same time, the rigidity of the set impression materials mixed with tap water increased by approximately 10%. With Ypeen Premium, the setting time decreased significantly by 20.0 seconds (P<.001) with tap water, while the working time and rigidity did not change. The setting behavior of Ypeen was not significantly affected by water quality. Acceleration of the setting reactions and increase in impression rigidity depended (P<.001) primarily on concentration and valency of cations present in mixing water. Setting of irreversible hydrocolloid impression materials can be significantly accelerated when tap water with high water hardness is used for mixing or when the mixing water contains higher concentrations of cations such as Na(+), Ca(2+), and Al(3+).

Electrodialytic removal of nitrates and hardness from simulated mixtures using ion‐exchange membranes

AbstractA novel electrodialysis (ED) pilot plant unit coupled with a membrane stack containing 11 cation‐exchange and 10 anion‐exchange membranes is used for the removal of nitrates and hardness from simulated aqueous mixtures containing salts that are usually encountered in brackish water. The removal of high nitrates and water hardness is performed in 150 min of ED under three constant applied voltages at room temperature. The limiting current density is obtained for sodium nitrate and calcium chloride mixtures in dilute solution. In order to check the efficacy of the ED method, parameters like the applied potential are varied at constant flow rates. The efficiency of the ED method depends on the applied potential. Possible applications of ED are discussed for the removal of contaminants below the minimum contaminant level of drinking water. The ED method used here is satisfactory to produce good quality drinking water from a simulated mixture by removing the unwanted ions. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 98: 1788–1794, 2006

Copper and Zinc Accumulation by a Transplanted Bivalve, Elliptio buckleyi, in Freshwater Systems in Central Florida

ABSTRACT We evaluated the potential use of the native freshwater bivalve, Elliptio buckleyi, as a biomonitor for stormwater metals [i.e., copper (Cu) and zinc (Zn)] in four small water bodies in central Florida. Mussels were collected from a local stream, placed into cages and then deployed into water bodies that received urban drainage of varying quality. Over a 12-month period, mussel growth rates and changes in tissue metal concentrations were compared to the reference site. Growth rate of E. buckleyi was not influenced by water column metal levels; initial mussel size largely determined growth rate. Growth rates were also greater at sites with higher water hardness and chlorophyll a concentrations, suggesting that these parameters may influence growth rates. Metal concentrations in soft-body tissues of transplanted mussels populations increased with increasing water column Zn and Cu concentrations and this relationship was independent of mussel growth rate. Our data demonstrate that E. buckleyi can be an effective biomonitor for these metals in fresh waters.

Water Self‐Softening Processes at Waterfall Sites

Abstract Many rivers in tropical and subtropical karst regions are supersaturated with respect to CaCO3 and have high water hardness. After flowing through waterfall sites, river water is usually softened, accompanied by tufa formation, which is simply described as a result of water turbulence in fast‐flowing water. In this paper, a series of laboratory experiments are designed to simulate the hydrological conditions at waterfall sites. The influences of air‐water interface, water flow velocity, aeration and solid‐water interface on water softening are compared and evaluated on a quantitative basis. The results show that the enhanced inorganic CO2 outgassing due to sudden hydrological changes occurring at waterfall sites is the principal cause of water softening at waterfall sites. Both air‐water interface area and water flow velocity increase as a result of the “aeration effect”, “low pressure effect” and “jet‐flow effect” at waterfall sites, which greatly accelerates CO2 outgassing and therefore makes natural waters become highly supersaturated with respect to CaCO3, consequently resulting in much CaCO3 deposition and reduction of water hardness. Aeration, rapidly increasing air‐water interface area and water flow velocity, proves to be effective in reducing water hardness. This study may provide a cheap, safe and effective way to soften water.

Calcium as a limiting resource to insectivorous bats: can water holes provide a supplemental mineral source?

Data are presented on a bat assemblage captured among 10 water holes in Colorado over 5 years. The assemblage consists of Myotis ciliolabrum, M. evotis, M. lucifugus, M. thysanodes, M. volans, Eptesicus fuscus, Lasiurus cinereus, Lasionycteris noctivagans and Corynorhinus townsendii. Results show that reproductive females and juveniles are captured in higher frequencies at water holes containing higher water hardness and that water hardness correlates highly significantly with dissolved calcium content. Also presented are laboratory test data on the stomach volume of Eptesicus fuscus that provide a model for understanding the effect of dissolved calcium content in water as a significant resource. These data indicate that water holes provide supplemental sources of calcium for bats not provided by diet.

Effects of Calcium Present in Fabric and in Washing Liquor on Detergency.

Measurement of the amount of calcium present in cotton fabrics, evaluation of the individual calcium contents in the actual washing liquors for the several washing conditions and several detergency tests were conducted, to clarify how calcium existing not only in water used for washing but also in cotton fabrics to be washed does affect on detergency. Calcium content in cotton fabrics was found to range from 80 to 600μg/g·fabrics. When the water hardness is low, the ratio of the calcium content in the fabrics to the total calcium content is high, so that the extent of influence caused by calcium present in the fabrics on detergency is relatively high. The detergency tests made clear that the calcium content in cotton fabrics has no influence on detergency at the higher water hardness, but it does have a negative influence at the lower water hardness.

Prevalencia y factores de riesgo en Yucatán, México, para litiasis urinaria

To measure the prevalence of urolithiasis in Yucatan, and to establish a possible association of the disease with family history and water hardness. During 1996, a survey was conducted on the population over one year of age in the State of Yucatan. Subjects were classified as having definite lithiasis (DL), probable lithiasis (PL), or no lithiasis (NL). Total hardness, calcium hardness, and magnesium hardness, were determined in drinking water samples. From a total of 5 832 questionnaires, 323 subjects (5.5%) were found to have DL, and 282 (4.8%) PL. The prevalence of lithiasis increased sharply with age, ranging from 1% in the population 18 years or younger, to 11.3% in those over 50 years. A family history of urolithiasis was positive in 44% of subjects with DL and in 34 % of those with PL, compared to 28% of those with NL (OR=2.8, 95% CI=2.2-3.4, p<0.0001, and OR=1.8, 95% CI=1.4-2.4, p<0.0001, respectively). Water hardness above 400 ppm was associated with a higher prevalence of urolithiasis (OR=1.9, 95% CI=1.21-2.94, p=0.003). The prevalence of urolithiasis in Yucatan is higher than that reported elsewhere in the scientific literature. A family history of the disease and high water hardness are risk factors that need to be investigated in future studies.

Ion chromatography determination of trace level bromate by large volume injection with conductivity and spectrophotometric detection after post column derivatisation

Bromate is a well known by-product produced by the ozonisation of drinking water; the allowed concentration for human consumption has to be regulated to the low μg l −1 range. A direct injection, ion chromatographic method was developed using a tetraborate eluent with serially connected conductivity and spectrophotometric detection. Bromate was detected after post-column reaction with fuchsin at 520 nm. Sample capacity was investigated by injecting large volumes (up to 6 ml) using a high total hardness and chloride tap water. Linear correlation of bromate response with volumes from 1 ml to 6 ml was demonstrated, the main limitation being the overlapping of the chloride peak with bromate. Up to 1.5 ml sample can be injected without any pre-treatment. With more than 1.5 ml injection volume, a sample pre-treatment with a cartridge in Ag and H form, followed by a 10 min degassing in an ultrasonic bath, was needed. This method was validated by analysing secondary reference materials and real samples from a drinking water treatment plant. The method was linear from the limit of quantification to 20 μg l −1. Reproducibilities in tap water were 18% (5 μg l −1, n=12) and 21% (1 μg l −1, n=4) respectively for 1.5 and 6 ml injection volumes with conductivity detection, and 17% at 0.5 μg l −1 ( n=9) with spectrophotometric detection. Calculated detection limits were 0.5 μg l −1 (6 ml) and 2 μg l −1 (1.5 ml) for conductivity detection and 0.3 μg l −1 (1.5 ml) for spectrophotometric detection.

Preparation of silicon oxide films having a water-repellent layer by multiple-step microwave plasma-enhanced chemical vapor deposition

Preparation of silicon oxide films having a water-repellent surface has been succeeded by means of a multiple-step microwave plasma-enhanced chemical vapor deposition (PECVD) process. The maximum water repellency of 110° was attained. In the first step of coating, a silicon oxide layer was prepared using a mixture of an organosilicon compound, that is, tetramethylsilane or tetramethoxysilane, and oxygen as a source gas. In the second step, oxygen was replaced with FAS-17 [(heptadecafluoro-1,1,2,2-tetrahydro-decyl)-1-trimethoxysilane]. We started to introduce FAS-17 using Ar carrier gas into the reactor and to reduce gradually supplying oxygen. Final step, in the absence of oxygen supply, the surface was further treated in the plasma for 1–10 min after the partial pressure of TMS or TMOS was adjusted so as to be equal to that of FAS-17/Ar. The films prepared by this process had high water repellency, hardness and transparency. The adhesion between the silicon oxide and the water-repellent layers was good due to the existence of the middle layer which consisted of inorganic and organic compositions.

Effect of acclimation period on the relationship of acute copper toxicity to water hardness for fathead minnows

AbstractThe acute (96‐h) toxicity of copper sulfate to fathead minnows was tested in a 2 × 2 factorial experiment, with factors being low (ca. 1 meq/L) and high (ca. 4 meq/L) water hardness during an acclimation period and low and high hardness during the exposure period. Acclimation hardness was found to have no significant effects on copper lethality, except that the 24‐h LC50 at high exposure hardness was 44% higher for fish subject to low acclimation hardness than fish subject to high acclimation hardness. In contrast, exposure hardness was found to have major effects, with LC50s being two to three times greater at high exposure hardness than at low, regardless of the acclimation hardness. These results suggest that the extent of the acclimation period is not a general, severely confounding factor for previous reports of the effects of hardness on copper toxicity to fish, although it may assume some importance for some species and test conditions.

A survey of commercial laundry detergents ‐ how effective are they?: Part III: powders, liquids and hard water

The effectiveness of 42 detergents, 11 non‐phosphate containing powdered detergents, 12 phosphate containing powdered detergents, 11 unbuilt liquid detergents and eight built liquid detergents, of varying formulations, in cleaning a standard soiled fabric in water of varying hardness was evaluated. Powdered detergents were found to be significantly affected only at very high water hardness levels. There was no significant difference between washing in warm and hot water for the liquid detergents, except for a melaleucaoil based detergent which performed significantly better in hot water. Increasing water hardness had no significant effect on liquid detergents. Powdered detergents performed better than liquid detergents in cleaning the standard soiled fabric. While no one detergent was significantly better than the others, in general, washing with detergents that contained phosphates did give somewhat better results in warm water. Detergents that contained bleach as an additive did not result in whiter fabric.

Toxicity of cobalt to freshwater indicator species as a function of water hardness

Although generally rare in natural aquatic systems, cobalt concentrations can be elevated through ore and coal mining operations and discharges of certain textile dyes. The U.S. Environmental Protection Agency does not have an ambient water quality criterion for cobalt due to a lack of toxicological data. The present study determined whether freshwater acute and chronic cobalt toxicity were dependent on dilution water hardness. We specifically examined the relative toxicity of cobalt at high water hardness (≥ 200 mg/l as CaCO 3) because cobalt often occurs in carbonate ores. Acute and chronic toxicity tests using both Ceriodaphnia dubia and Pimephales promelas were performed using four different synthetic waters: 50,200. 400, and 800 mg/l hardness as CaCO 3. Diluent hardness appeared to have no effect on measured cobalt concentrations. Dissolved cobalt in water samples during testing was similar to the corresponding total recoverable cobalt in this study suggesting that most of the cobalt was in a soluble and presumably bioavailable form. LC 50 values for P. promelas could not be calculated owing to their low sensitivity to high cobalt concentrations (≥ 5 mg/l) over a 48 h period. Regression analysis of P. promelas acute NOEC values and test water hardness indicated an R 2 = 0.94 suggesting a direct relationship between water hardness and acute toxicity to cobalt for this species. C. dubia acute tests indicated water hardness effects after 48 h of exposure and non-linear dose responses in hard water (≥ 200 mg/l). Regression analysis of C. dubia 24 h LC 50 values and test water hardness indicated an R 2 = 0.91. For both cases, a simple power function gave the best regression fit. C. dubia appeared to be more sensitive than P. promelas at hardness ≥ 200 mg/l but the reverse was true in soft water ( ≤ 50 mg/l). The 7 d P. promelas chronic NOEC increased from 1232 μg/l cobalt (50 mg/l hardness) to ≥ 3833 μg/ l (800 mg/l hardness). Effects were on fish survival and not growth. Regression analysis yielded an R 2 = 0.97 based on the fish survival NOECs and the natural log of hardness. C. dubia chronic tests were inconclusive due to poor survival and reproduction in the synthetic waters. Available data suggest that within 50–200 mg/l water hardness, cobalt acute toxicily is inversely related to water hardness. Tentative cobalt acute criteria, based on power functions, are 288 μg/l and 873 μg/l for soft and hard water, respectively. Available chronic toxicity data suggest that daphnids are more sensitive to cobalt than other genera. Chronic endpoints for both P. promelus and D. magna were similar in soft and hard water (≤ 400 mg/l hardness) suggesting that chronic toxicity is not hardness-dependent over this range. However, chronic cndpoints at 800 mg/l hardness were 3.1 and ≥ 12 times higher than endpoints observed under soft water conditions for P. promelas and C. dubia respectively suggesting that very hard waters may significantly reduce chronic toxicity potential of cobalt.

The effects of water hardness upon the uptake, accumulation and excretion of zinc in the brown trout, Salmo trutta L.

The effects of water hardness (9 and 220 mgl−1 as CaCO3) upon zinc exchange in brown trout exposed to 0.77 μmol Zn 1−1 have been investigated using artificial soft water (&lt;49.9 μmol Ca l‐1, &lt;40.1 μmol Mg 1−1) and mains hard water (1671.7 μmol Ca 1−1, 493.6 μmol Mg 1−1) of known composition. Both hard and soft water‐adapted fish exhibited a bimodal pattern of net zinc influx. Net zinc influxes during both fast and slow uptake phases were significantly greater (P&lt;0.001) in soft (82.9 and 6.2 μmol Zn 100 g−1 h−1) than in hard water (46.3 and 2.4 μmol Zn 100 g h−1). Zinc efflux (‐ 0.2 μmol Zn 100 g−1 h−1) was enhanced only in hard water during the slow net influx phase.Brown trout exposed to zinc in hard water and placed in metal‐free media exhibited a greater net efflux (‐ 25.6 μmol Zn 100 g−1 h−1) of the metal than did fish in soft water (‐4.2 μmol Zn 100 g−1 h−1) treated in the same manner. Tissue 65Zn activities reflected both the differences in uptake and excretion rates of the metal between hard and soft water fish. During zinc exposure (0.77 μmol Zn 1−1) high water hardness reduced tissue burdens of the metal by reducing net branchial influx, and enhancing efflux of the metal in hard water fish.