Modeling Seasonal Arsenic Behavior in the Waikato River, New Zealand

Chapter 18

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Modeling Seasonal Arsenic Behavior in the Waikato River, New Zealand Jenny G . Webster-Brown and Vincent Lane S G E S Environmental Science, University of Auckland, P.O. Box 92019, Auckland, New Zealand

Abstract Significant seasonal variations occur in As concentrations in the Waikato River, a major lowland river in New Zealand. The As is of geothermal origin, and an acceptable explanation for the seasonal change has proved elusive. Data for As and Li concentrations from a monthly river water quality monitoring program, together with data from a recent study of suspended particulate material (SPM) and As in the lower catchment, have provided the basis for a new hypothesis; that the summer maximum in As concentration represents conservative behavior of the geothermal As from the upper catchment, and the winter minimum is due to the combined effects of greater dilution of geothermal fluid flow in river waters, and a greater degree of As adsorption onto Fe oxide-rich, winter S P M . To test this hypothesis, in-river and experimental As adsorption onto S P M has been modeled using a diffuse layer, surface complexation model, assuming Fe oxide to be the only adsorbing surface present. The model was able to reproduce the summer and winter seasonal variations in the degree of As adsorption onto SPM. However, during algal blooms in late summer and autumn, biological uptake by diatoms may also result in As removal from the water column.

© 2005 American Chemical Society

In Advances in Arsenic Research; O'Day, P., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2005.

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Introduction The Waikato River is New Zealand's longest river, and is extensively used for industry, domestic, pastoral and agricultural water supply. The upper catchment includes a large area of natural geothermal activity, together with several geothermal power stations and 8 hydroelectric lakes (Figure 1). In the middle reaches of the river, near Hamilton city, the concentration of As in the Waikato River water typically ranges from 0.02 to 0.06mg/L (/). In the lower river, near Tuakau, the concentrations typically range from 0.01- 0.03mg/L (2). The As is predominantly present in dissolved form (73-100%; 2), as inorganic A s (J), and is entirely of geothermal origin (4). A number of research studies have been undertaken into the behavior, toxicity and bioavailability of As in this river system (e.g., /, 2, 3, 5, 6, 7, 8). One aspect of As behavior that has so far defied conclusive explanation is the distinct seasonal trend, which shows a broad maximum in As concentration in summer and a minimum in winter (Figure 2). Various hypotheses have been proposed for this seasonal trend, including; v

a)

Desorption of As from sediments and suspended particulate matter (SPM) during the summer months, as a direct effect of temperature on the A s adsorption process (e.g., /). The temperature typically ranges from 10-25°C. This direct effect has been effectively disproved in adsorption experiments (2), where little difference was observed in A s adsorption onto S P M over the temperature range 5 - 30°C. v

v

b)

v

1

Abiotic or biotic reduction of sedimentary A s to As ", which is not as readily adsorbed onto sediment, resulting in As release into the water column (e.g., /). A review of thermodynamic data for As redox reactions indicated that a temperature increase of ~15°C would be unlikely to significantly increase abiotic reduction of A s . However, bacteria capable of reducing A s have been previously identified in the Waikato River and their activity was observed to be notably higher in summer {12). High concentrations of As in the water column have been detected periodically in the upper reaches of the river (e.g., 5, 13). However, fortnightly monitoring of dissolved AsV and AsIII throughout the lower catchment (at Tuakau) in 2003 by the authors showed that As " did not exceed 10% of the dissolved As at any time, and was in fact lowest (0.7%) during the As maximum (0.038 mg/L), which occurred in May in 2003. v

v

111

1

c)

1

Periodic release of the As " present in the anoxic basal layer of the stratified hydroelectric lakes, when the lake turnover occurs in autumn (7). This would result in a very short-term pulse of high As concentration in the water column, but does not appear to constitute an explanation for the consistently higher As concentrations throughout summer and autumn.



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Figure 1. The Waikato River catchment, North Island, New Zealand, showing the positions of major geothermal fields and hydroelectric dams.

Figure 2. Seasonal changes in total As concentration in the lower Waikato River at Tuakau (9, 10, 11). Shaded areas = summer and autumn months.



256 Given the inability of each of these proposed hypothesis to fully explain As seasonal behaviour, a re-evaluation of As behavior in this river system has been undertaken using monthly chemical monitoring data for the Waikato River, collected over the last 10 years by the regional council "Environment Waikato" (e.g., 10% and the results of a more detailed study of As and S P M at Tuakau (2). The assumption inherent in all of the previous hypotheses, that the summer maximum is in some way due to As release from the river sediments, is challenged. Instead it is proposed that the summer maximum represents conservative behavior of dissolved As at this time of year. The winter minimum is caused by a greater degree of geothermal fluid dilution in the higher river flows of winter, combined with greater As adsorption onto the more Fe oxiderich S P M which occurs after the algal growth in summer and autumn has ended. To test this hypothesis, in-river and experimental As adsorption onto S P M has been modeled using a diffuse layer, surface complexation model, assuming Fe oxide to be the only adsorbing surface present.

Seasonal changes in As concentrations and SPM composition Arsenic concentration Monthly water quality monitoring data for L i and "total" As concentration have been used in this analysis of seasonal variability. Sampling and analytical methods are outlined in detail in Wilson (70); L i has been determined on an unfiltered water sample by ICP-mass spectrometry (ICP-MS) and "total" As on an unfiltered water sample subjected to a nitric acid digestion, also by ICP-MS. Some degree of seasonal variation in As concentration might be expected as a direct consequence of changing river flow rate, with greater dilution of the geothermal fluid input in winter when there is a higher rainfall. Indeed, monitoring data for other geothermal chemicals such as L i and B , which are known to behave conservatively, do show summer maxima and winter minimum due to dilution (e.g., data in 10). However, the variation in As concentration is more extreme. This is evident in the seasonal change in the ratio of total As:Li (Figure 3). The weight ratio of total As:Li immediately downstream of the largest geothermal fluid input (at Wairakei) is relatively constant at 0.29 ± 0.02 (average and standard deviation for combined monthly data for 1998, 1999 and 2000). As shown in Figure 3, in summer this ratio persists into the lower reaches of the river (at Hamilton and Tuakau), with neither As nor L i being preferentially removed from or added to the water column. Similar trends in the As:Li ratio were evident in the data for every year since monitoring data collection commenced in 1993. Consequently, in summer the concentrations of As reflect conservative behavior. Dilution of geothermal fluid in higher river flows in winter would affect both ions to the same degree. Therefore, if higher rainfall and higher river flow



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were responsible for the winter minimum, the As:Li ratio would remain more-orless unchanged from summer to winter. However, this is clearly not the case, as the As:Li ratio decreases significantly over winter (Figure 3), suggesting a greater degree of removal of As from the water column. Chemical conditions in the river do not favour precipitation of As-bearing minerals, so the most likely mechanisms for As removal are uptake by biota or adsorption onto particulate material. Arsenic accumulation by aquatic plants has been reported from this river (5, 8). However, the maximum growth period for plants, when they can remove dissolved As most effectively, is in spring and summer, rather than in winter. Arsenic uptake by phytoplankton is also a recognized phenomenon (e.g., 14), but once again diatom growth occurs predominantly in summer and autumn, rather than in winter. Therefore As removal by adsorption onto abiotic particles, particularly adsorption onto the reactive S P M in the water column, appears to be the most viable option. Significantly, there appears to be no consistent seasonal change in factors affecting As adsorption such as pH (typically within the range 7.2-7.9 at Tuakau; 10) or major ion concentrations (15).

Figure 3. The weight ratio of total As: Li in the lower Waikato River, at Tuakau (o) and Hamilton (·) in 1999 (data from (10)). The shaded area shows the ratio (±SD) present in the upper Waikato River, immediately below Wairakei but upstream of the hydroelectric lakes.


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Composition of S P M A more detailed study of Waikato River chemistry, including As, other trace element and S P M concentrations, and S P M characterization, was undertaken at Tuakau from November 1998 - November 1999. Details of sampling and analytical methods are given in Webster-Brown et al., (2). The standard, arbitrary definitions of "dissolved" as measured on a water sample after field filtration through a 0.45μηι membrane, "acid-soluble" as measured in an unfiltered water sample acidified to pH

Modeling Seasonal Arsenic Behavior in the Waikato River, New Zealand

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