P L. Smedley, D G. Kinniburgh/Applied Geochemistry 17(2002 )517-568 n htrations up to 370 Hg I-l in Madison River water most of the catchment was in the range 0.75-38 HeA B- found. Nimick et al. (1998) for example found As con- sewage. However, the concentration of As in water fro yoming and Montana) as a result of geothermal and not significantly different from baseline concentra inputs from the Yellowstone geothermal system. wilkie tions. Durum et al. (1971)reported As concentrations in nd Hering(1998)also found concentrations in the 727 samples of surface waters from the United States range 85-153 ug I-I in Hot Creek(tributary of the While 79% of the samples had As concentrations below Owens River, California). Some river waters affected by the(rather high) detection limit of 10 ug l-l, the highest geothermal activity show distinct seasonal variations in observed concentration, 1 100 ug I-l, was found in Sugar As concentration Concentrations in the madison river Creek. South Carolina. downstream of an industrial have been noted to be highest during low-flow condi- complex tions. This has been attributed to a greater contribution Arsenic can also be derived from mine wastes and mill of geothermal water during times of low flow and dilu- tailings. Azcue and Nriagu (1995) found baseline con tion from spring runoff at times of high fow(Nimick et centrations in the Moira River, Ontario of 0.7 ug aL., 1998). In the Waikato river system of New Zealand, upstream of the influence of tailings from gold-mine As maxima were found in the summer months. These workings. Downstream, concentrations increased to 23 increases were linked to temperature-controlled micro- Hg l-l. Azcue et al. (1994)found concentrations up to bial reduction of As(v) to As(Ill with consequent 556 ug l-(average 17. 5 ug l-)in streams adjacent to increased mobility of As(lID(McLaren and Kim, 1995) posits in British Columbia. Williams et al. Increased concentrations are also reported in some (1996)and Smedley et al.(1996) noted high As con river waters from arid areas where the surface water centrations(typically around 200-300 ug 1-)in surface dominated by river baseflow. The resulting surface waters affected respectively by Sn- and Au-mining activ- waters often have a high pH and alkalinity. For example, ities. Though often involving notable increases above in surface waters from the loa river basin of northern baseline concentrations such anomalies tend to be rela Chile( antofagasta area, Atacama desert), Caceres et al. tively localised around the pollution source, principally ( 1992) found concentrations of naturally-occurring As because of the strong adsorption affinity of oxide miner ranging between 190 and 21, 800 ug I-. The high As als, especially Fe oxide, for As under oxidising, neutral concentrations correlated well with salinity. While geo- to mildly acidic conditior hermal inputs are likely to have had an importan impact on the chemical compositions of the river waters 2. 2.3. Lake water in this area(Section 5.5), evaporative concentration of Concentrations of As in lake waters are typica baseflow-dominated river water is also likely to be to or lower than those found in river water important in the arid conditions. Increased As con- concentrations have been found at <l ug I- centrations(up to 114 ug l-)have also been reported in (Azcue and Nriagu, 1995; Azcue et al., 1995). As with river waters from central Argentina where regional river waters, increased concentrations are found in lake groundwater-As concentrations(and pH, alkalinity) are waters affected by geothermal water and by mining high(Lerda and Prosperi, 1996). activity. Ranges of typically 100-500 ug I-I have been Although bedrock inevitably has an influence on reported in some mining areas and up to 1000 ug l-in iver-water As concentrations, concentrations in rivers geothermal areas ( Table 1). Arsenic concentrations in ith more typical pH and alkalinity values(c pH 5-7, mining-affected lake waters are not always high how alkalinity <100 mg I-I as HCO3)do not show the extre- ever, as removal from solution can be achieved effec mely high concentrations found in groundwaters because tively by adsorption onto Fe oxides under neutral to of oxidation and adsorption of As species onto the river mildly acidic conditions. Azcue et al.(1994), for exam- sediments as well as dilution by surface recharge and run- ple, found As concentrations in Canadian lake waters off. Arsenic concentrations in seven river water samples affected by mining effluent similar to those not affected from Bangladesh have been reported in the range <0.5-2.7 by mining effluent, in each case about 0.3 ug I ug I- but with one sample having a high concentration of High As concentrations are also found in some alk 29 ug I(BGS and DPHE, 2001). The highest value line closed-basin lakes as a result of extreme evapora observed is significantly above world-average baseline tion and/or geothermal inputs. Mono Lake in the concentrations (Table 1)but is much lower than some of California, USA, for example, has concentrations of he values found in the groundwaters dissolved As of 10,000-20,000 ug I-, with pH values in Significant increases in As concentrations of river the range 9.5-10 as a result of inputs from geothermal waters may also occur as a result of pollution from springs and the weathering of volcanic rocks followed industrial or sewage effluents. Andreae and Andreae by evaporation(Maest et al., 1992) ( 1989)found concentrations up to 30 ug I-in water There is also much evidence for stratification of as from the River Zenne, Belgium which is affected by concentrations in some lake waters as a result of varying inputs from urban and industrial sources, particularl redox conditions(Aggett and o Brien, 1985). Azcue andfound. Nimick et al. (1998) for example found As con￾centrations up to 370 mg l
1 in Madison River water (Wyoming and Montana) as a result of geothermal inputs from the Yellowstone geothermal system. Wilkie and Hering (1998) also found concentrations in the range 85–153 mg l
1 in Hot Creek (tributary of the Owens River, California). Some river waters affected by geothermal activity show distinct seasonal variations in As concentration. Concentrations in the Madison River have been noted to be highest during low-flow condi￾tions. This has been attributed to a greater contribution of geothermal water during times of low flow and dilu￾tion from spring runoff at times of high flow (Nimick et al., 1998). In the Waikato river system of New Zealand, As maxima were found in the summer months. These increases were linked to temperature-controlled micro￾bial reduction of As(V) to As(III) with consequent increased mobility of As(III) (McLaren and Kim, 1995). Increased concentrations are also reported in some river waters from arid areas where the surface water is dominated by river baseflow. The resulting surface waters often have a high pH and alkalinity. For example, in surface waters from the Loa River Basin of northern Chile (Antofagasta area, Atacama desert), Ca´ceres et al. (1992) found concentrations of naturally-occurring As ranging between 190 and 21,800 mg l
1 . The high As concentrations correlated well with salinity. While geo￾thermal inputs are likely to have had an important impact on the chemical compositions of the river waters in this area (Section 5.5), evaporative concentration of baseflow-dominated river water is also likely to be important in the arid conditions. Increased As con￾centrations (up to 114 mg l
1 ) have also been reported in river waters from central Argentina where regional groundwater-As concentrations (and pH, alkalinity) are high (Lerda and Prosperi, 1996). Although bedrock inevitably has an influence on river-water As concentrations, concentrations in rivers with more typical pH and alkalinity values (c. pH 5–7, alkalinity <100 mg l
1 as HCO3) do not show the extre￾mely high concentrations found in groundwaters because of oxidation and adsorption of As species onto the river sediments as well as dilution by surface recharge and run￾off. Arsenic concentrations in seven river water samples from Bangladesh have been reported in the range <0.5–2.7 mg l
1 but with one sample having a high concentration of 29 mg l
1 (BGS and DPHE, 2001). The highest value observed is significantly above world-average baseline concentrations (Table 1) but is much lower than some of the values found in the groundwaters. Significant increases in As concentrations of river waters may also occur as a result of pollution from industrial or sewage effluents. Andreae and Andreae (1989) found concentrations up to 30 mg l
1 in water from the River Zenne, Belgium which is affected by inputs from urban and industrial sources, particularly sewage. However, the concentration of As in water from most of the catchment was in the range 0.75–3.8 mg l
1 and not significantly different from baseline concentra￾tions. Durum et al. (1971) reported As concentrations in 727 samples of surface waters from the United States. While 79% of the samples had As concentrations below the (rather high) detection limit of 10 mg l
1 , the highest observed concentration, 1100 mg l
1 , was found in Sugar Creek, South Carolina, downstream of an industrial complex. Arsenic can also be derived from mine wastes and mill tailings. Azcue and Nriagu (1995) found baseline con￾centrations in the Moira River, Ontario of 0.7 mg l
1 upstream of the influence of tailings from gold-mine workings. Downstream, concentrations increased to 23 mg l
1 . Azcue et al. (1994) found concentrations up to 556 mg l
1 (average 17.5 mg l
1 ) in streams adjacent to tailings deposits in British Columbia. Williams et al. (1996) and Smedley et al. (1996) noted high As con￾centrations (typically around 200–300 mg l
1 ) in surface waters affected respectively by Sn- and Au-mining activ￾ities. Though often involving notable increases above baseline concentrations, such anomalies tend to be rela￾tively localised around the pollution source, principally because of the strong adsorption affinity of oxide miner￾als, especially Fe oxide, for As under oxidising, neutral to mildly acidic conditions. 2.2.3. Lake water Concentrations of As in lake waters are typically close to or lower than those found in river water. Baseline concentrations have been found at <1 mg l
1 in Canada (Azcue and Nriagu, 1995; Azcue et al., 1995). As with river waters, increased concentrations are found in lake waters affected by geothermal water and by mining activity. Ranges of typically 100–500 mg l
1 have been reported in some mining areas and up to 1000 mg l
1 in geothermal areas (Table 1). Arsenic concentrations in mining-affected lake waters are not always high how￾ever, as removal from solution can be achieved effec￾tively by adsorption onto Fe oxides under neutral to mildly acidic conditions. Azcue et al. (1994), for exam￾ple, found As concentrations in Canadian lake waters affected by mining effluent similar to those not affected by mining effluent, in each case about 0.3 mg l
1 . High As concentrations are also found in some alka￾line closed-basin lakes as a result of extreme evapora￾tion and/or geothermal inputs. Mono Lake in the California, USA, for example, has concentrations of dissolved As of 10,000–20,000 mg l
1 , with pH values in the range 9.5–10 as a result of inputs from geothermal springs and the weathering of volcanic rocks followed by evaporation (Maest et al., 1992). There is also much evidence for stratification of As concentrations in some lake waters as a result of varying redox conditions (Aggett and O’Brien, 1985). Azcue and 524 P.L. Smedley, D.G. Kinniburgh / Applied Geochemistry 17 (2002) 517–568