Arsenic in Swedish groundwater Mobility and risk for naturally elevated concentrations : Final Report

• Arsenic (As) in groundwater has become a serious global problem during the last few decades forseveral reasons. The chronic toxicity from drinking water has motivated the lowering of thehealth limit from 50 μg/L to 10 μg/L. Due to the rather complicated analytical chemistry of As ithas seldom been analysed in groundwater. Once the common occurrence of excess As ingroundwater has been discovered like in Bangladesh it has become evident that As in concentrationswell above the health limit can be easily mobilized from very moderate amounts in the aquifermaterial under specific conditions. There are essentially three mechanisms of mobilization: 1)oxidation of sulphides containing As; 2) reduction of ferric compounds releasing adsorbed Asand 3) high pH conditions leading to lowered adsorption capacity of ferric and aluminium compounds. This study has aimed at investigating the mobilization of As under the conditions existing in Precambrianrocks and the overlying tills in Sweden. As study area the north-eastern part of theVästerbotten county was chosen. The occurrence of sulphides both in mineralisations and insome of the country rocks may infer that the risk of elevated As concentrations in groundwatermay be a bit higher there than elsewhere in Sweden. The hypothesis was that the two firstmechanisms mentioned above would be responsible for any elevated groundwater As in groundwater,thus either oxidation of sulphides or reducing conditions leading the reduction of ferrichydroxides. Samples were collected from drilled wells, dug wells and springs. pH, Eh and temperature weremeasured in the field and the samples were filtered through 0.20 m filters in the field. As(II) wasseparated from As(V) in the field by ion-exchangers. As redox conditions are the determinantsfor the mobilization of As the groundwaters have been classified into five redox classes followinga classification developed by the Swedish Environmental Protection Agency. Dug wells with elevatedAs turned out to be in the oxidizing part of the classification while drilled wells were foundin all classes. This is reasonable as drilled wells often collect water from different environmentsthrough the different fracture systems the drilled wells contact. Wetland springs are found tohave moderately reducing water high in iron. While drilled wells and dug wells show a wide spectrumof As(III)/As(tot) ratios the As(III) is clearly dominant in wetland springs. Drilled wellsshowed the highest concentrations of As up to 300 μg/L and wells drilled in alkaline volcanicrocks had the highest median concentrations of As. Arsenic in wetland springs were strongly correlatedto iron. Another clear relationship was that groundwater with even low concentrations ofnitrate was low in As. Obviously the presence of nitrate indicates an oxidizing environmentwhere As is firmly adsorbed onto ferric hydroxides. There was no clear relationship with sulphateindicating that the drilled wells which made up the majority of the groundwaters often had waterof a mixed origin coming to the well via different fracture systems. The results indicate that As should in general be analysed in household wells as it is difficult topredict the presence of elevated concentrations from the major ion chemistry. High iron concentrationis a factor that indicates an elevated risk of As above the health limit. Nitrate on the otherhand indicates a low risk for elevated As concentration.

Subject headings

NATURVETENSKAP  -- Geovetenskap och miljövetenskap (hsv//swe)

NATURAL SCIENCES  -- Earth and Related Environmental Sciences (hsv//eng)

Keyword

arsenic

groundwater

basement rocks

redox classes

hydrochemistry

borewells

dugwells

Västerbotten County

Sweden

Publication and Content Type

vet (subject category)

rap (subject category)