Mats E. Åström

Impact of acid sulfate soils on stream water geochemistry in western Finland

Abstract The influence of acid sulfate soils on stream water geochemistry was studied in thirteen catchment basins in western Finland. Seventy-four unfiltered stream water samples were collected in October 1991 and were analyzed for 29 elements. The occurrence of clay/silt sediments in the catchments was used as an estimate of the occurrence of acid sulfate soils, because the development of acid sulfate soil is extensive from this overburden type. The median pH of the water samples was found to be low, and the median concentrations of several elements, including Al, Co, Li, Mn and Ni, are much higher than those in headwater streams of Finland and in Fennoscandian rivers. High correlation coefficients were found between the percentage of clay/silt sediments in the catchments and the concentrations of Al, B, Be, Ca, Cd, Co, Ni, K, Li, Mg, Mn, Na, (SO 2− 4 ), Sr, U and Zn in the stream waters, indicating extensive leaching of these elements from the acid sulfate soils. There are also indications of stronger leaching of Cr, Si and Tl from the acid sulfate soils than from other soils. The concentrations of As, Fe, Pb, Sb and V vary independently of soil type. Only one element, V, shows a positive correlation with the content of total organic carbon, which was analyzed in 10 water samples.

Partitioning of transition metals in oxidised and reduced zones of sulphide-bearing fine-grained sediments

Partitioning of transition metals in reduced and oxidised zones of sulphide-bearing fine-grained sediments

Abundance and fractionation patterns of rare earth elements in streams affected by acid sulphate soils

Abstract The abundance and shale-normalised fractionation patterns of rare earth elements (REEs) were studied in 121 streams of first and second order in an area where acid sulphate soils, developed on oxidised sulphide-bearing marine sediments, are abundant. During high-water flow events in autumn, on which this study focuses, the leaching of REEs from the acid sulphate soils is extensive, resulting in high total REE concentrations (up to >1 mg/l) in streams draining such soils exclusively. In these streams, there is a general depletion of the heavy REEs explained by preferential scavenging of the latter by oxyhydroxides in the deepest soil-profile horizons. In streams draining areas of glacial till and peat, the REE concentrations are several orders of magnitude lower. In these streams, in contrast to the REE-enriched streams, there is a depletion of light REEs explained most likely by preferential complexation of the middle and heavy REEs by dissolved humic substances.

Spatial trends and losses of major and trace elements in agricultural acid sulphate soils distributed in the artificially drained Rintala area, W. Finland

Acid sulphate soils, common in the coastal areas of Finland, contribute strongly to high acid, S and metal loadings on adjacent surface waters. This, in turn, is causing significant harm to the aquatic ecology. There is, however, limited knowledge on the total amounts of acidity and chemical elements leached from these soils. The overall objective of this study was to determine geochemical patterns in acid sulphate soils and their parent sediments and, based on the identified patterns assess the extent, mechanisms and present state of leaching of major and trace elements from these soils. The distribution of pH, aqua regia extractable concentrations of P and metals (Al, Ba, Ca, Co, Cr, Cu, Fe, K, La, Mg, Mn, Na, Ni, Sr, Th, Ti, V, Zn) and total concentrations of S and C were determined in 30 vertical profiles collected in the 23 km2 large Rintala agricultural area (mid-western Finland) underlain largely with S-rich sediments. It was found that approximately 70% of the area consists of acid sulphate soils with a minimum pH 4%. Four different methods were used to estimate the losses of chemical elements from the acid sulphate soils: (1) the concentrations in the soil were compared with those in the parent sediments, (2) due to indicated heterogeneities in several profiles, the vertical changes of the immobile Ti was used to re-calculate element losses, (3) element depletions in the acid sulphate soils (as compared to those in the parent sediments) were compared to the corresponding depletions in the non acid sulphate soils, (4) element concentrations in drainage waters were compared with those in the parent sediments. Based on these calculations, it was assessed that the percentual leaching of the aqua regia extractable fraction (total for S) has been as follows: S (40–50%), Na (30–50%), Mn (25–35%), Sr (15–20%), Ca–Ni–Co (approximately 10%), Mg–K–Zn (5–10%), Th–La–Cu–Al–P–Ti–Fe (<5%), and Ba–Cr–V (<1%). While it was possible to quite accurately estimate the percentages and thus the amounts of elements lost, it was not possible to estimate the rate of leaching as there is no available detailed information on dates when ditching activities and thus oxidation-acidification processes started. Other calculations indicated that the mobile S reservoir is still some 15 ton/hectare, which is huge but still smaller than the losses that have occurred since the area was drained (23–28 ton/hectare).

Geochemistry of stream water in a catchment in Finland affected by sulphidic fine sediments

Abstract The impact of sediment type on stream water geochemistry was studied in a catchment in Finland affected by sulphidic fine-grained sediments. Stream water samples for general characterisation of water quality (pH, electrical conductivity) were taken at the basin outlet during various hydrological conditions, while samples for detailed geochemical analysis were collected at 119 sites in the catchment on one single occasion during high-water flow in autumn. The occurrence of sulphidic fine sediments was estimated based on data from an airborne electromagnetic survey carried out by the Geological Survey of Finland. Growing-symbol maps, which were prepared for each of the studied variables in water, and statistical calculations including factor analysis and Spearman correlations show that the concentrations of Al, Ga, U and Tl, all the lanthanides and several alkali and alkaline earth metals (K, Mg, Na, Li, Ca, Rb, Sr), transition metals (Cd, Co, Cu, Zn, Sc, Mn, Ni, Y, Hf) and non-metals (S, Br, I, Si) increase in water when the proportion of the catchment cover of sulphidic fine sediments increases. It is therefore argued that these elements are released and mobilised in considerable amounts by the oxidation and subsequent acidification and weathering of this type of sediment. Other elements are either slightly depleted in streams in areas of sulphidic fine sediments (V, Nb, Pb, Zr), have a distribution unrelated to sediment type (Fe, Cr, Cs, Mo), or are only weakly impacted by the occurrence of sulphidic sediments in the catchment (As, Ti, Ba). It is argued that these elements are not leached extensively from the oxidising sulphidic sediments, and that their distributions at least partly may be controlled by the contents of dissolved humic material and/or suspended organic and inorganic phases in the water.

Estuarine behaviour of metal loads leached from coastal lowland acid sulphate soils

The estuarine behaviour of the metal load leaching from acid sulphate (AS) soils was studied in a selected river system (the Vörå River), in western Finland. Large amounts of metals were transported with the river and deposited within the estuary, causing highly elevated metal concentrations in both the sediment traps and in the underlying bottom sediments. Among the metals, there was a diverging deposition pattern where Al, Cu, La and U demonstrated a strong association with organic matter and were deposited within approximately 4 km from the river mouth. In contrast, the deposition of Co, Mn, Ni and Zn occurred when pH reached circumneutral conditions further out in the estuary. Yet other metals were not abundantly leached from the AS soils and thus not elevated in the river and estuary (Fe, Ti, Cr, V). Five separate chemical extractions indicated the geochemical speciation of the metals.

Geochemistry and acidity of sulphide-bearing postglacial sediments of western Finland

The geochemistry, mineralogy, acidity and electric conductivity of sulphide-bearing postglacial sediments located on the coastal plains of western Finland were studied. Grain size and mineralogical analysis (eight samples studied) indicated that the sediments are silts dominated by primary minerals (quartz, feldspars), and that the clay fraction (<2µm), which made up less than one-third of the bulk samples, is enriched in phyllosilicates but depleted in quartz and feldspars, relative to bulk samples. As compared to the metal contents of the fine-fraction of glacial till (data of the Geological Survey of Finland), the fine sediments appear to be enriched in sulphur (median = 0.54%) and trace metals such as Ni (31 µg g−1), Zn (90 µg g−1), Co (13 µg g−1), Cr (48 µg g−1) and V (50 µg g−1). Statistical analysis indicated that aluminosilicates are the principal carrier phases of trace metals, and that sulphides and organic matter only partially control the distribution of elements other than S and C. A 3-g portion of 232 samples was oxidised in the laboratory by atmospheric oxygen for a period of 1 year. After this period of oxidation, 15 ml of water was added, after which pH and electric conductivity of the suspensions (sediment:water, 1:5) were measured. The quantities of elements mobilised by the oxidation was calculated from the dissolved concentrations found in 3 g of oxidised sediment suspended in 30 ml of deionised water (eight samples studied). The results of the experiment showed that the impact of sulphide oxidation on the pH and conductivity of the sediments is large, and that a high proportion (>13%) of the aqua-regia extractable contents of several elements (Co, Mn, Ca, Ni, Zn, Sr, Na and B) are mobilised at oxidation. Because of the high quantities of acidity and metals liberated at oxidation, it is argued that stream-water quality, which is poor in the study area, will not improve unless artificial draining is restricted in areas of sulphide sediments.

Abundance, Sources and Speciation of Trace Elements in Humus-Rich Streams Affected by Acid Sulphate Soils

The behaviour of trace elements (Al, As, Cd, Co, Cr,Cu, Fe, Mn, Ni, V, Zn) was studied in five humus-richstreams (dissolved organic carbon = 14–40 mg/L)impacted by acid sulphate soils developed in marinesulphide-bearing fine-grained sediments. During heavyrainfalls in autumn, on which the study focusses, themetals Al, Cd, Co, Cu, Mn, Ni and Zn are extensivelyleached from these acidic soils (pH = 2.5–4.5), whileAs, Cr, Fe and V are not leached more strongly fromthis soil type than from areas of till and peat. Aspeciation experiment, based on anion and cationexchange of the stream waters in the field, showedthat (1) the metals Al, Cd, Co, Mn, Ni and Zn aretransported in the streams mainly as inorganiccations, (2) Cu exists mainly in cationic form but isalso to a significant extent associated with dissolvedhumic substances, (3) Fe occurs mainly in the anionicfraction explained by organic coating on colloidal Feoxyhydoxides and (4) the hydrochemistry of As, Cr andV is complex as these elements may exist in severalunquantified anionic fractions and to a minor extentin cationic species/forms. Whereas the proportion ofacid sulphate soils in the catchments had a largeimpact on concentrations levels of several elements inthe stream waters, these soils did not have a largeaffect on the speciation of elements in water.

Quantification of current and future leaching of sulfur and metals from Boreal acid sulfate soils, western Finland

The leaching of sulfur (S) and metals (Al, Ca, Cd, Co, Cu, Fe, K, Mg, Mn, Ni, Zn) from an acid sulfate soil (ASS) area in western Finland was determined on the basis of hydrochemical analyses (ICP-MS) of water samples collected monthly for 3 years from the stream draining that area. The average annual amount of leaching was as follows (kg/ha.year): S (633), Ca (281), Mg (199), Al (54), K (54), Mn (35), Fe (5.6), Zn (1.7), Ni (0.84), Co (0.79), Cu (0.070), Cd (0.0068). These high values are due to extensive oxidation of metal sulfides and weathering of minerals in the ASS profile. Calculations showed that other S inputs such as deposition and fertiliser use, and S outputs such as degassing and plant removal, are insignificant in comparison with current leaching losses. Before the area was artificially drained, the leaching losses of S from the study area must have been very small; otherwise, the S residual in the soil would have been depleted a long time ago. With current drainage practices, the leachable soil S residual will be halved in roughly 30 years, after which the S and metal loads of the drainage will have decreased. However, more time is needed before the concentrations will have decreased to an environmentally acceptable level, unless environmentally friendly measures are found and implemented.

Hydrogeochemistry of a stream draining sulfide-bearing postglacial sediments in Finland

The influence of hydrological conditions and overburden type on the stream-water quality in a catchment effected by fine-grained sulfide-bearing sediments was studied. Water samples for measurement of pH and electric conductivity were collected at 16 sites along the main course of the stream during autumn and spring high water flow and during low and intermediate water flow. On one occasion (spring high-water flow) samples for analysis of total organic carbon were collected at these sites. Seventeen water samples for analyses of chemical elements (ICP-MS and ICP-AES) were taken within the catchment basin during autumn high water flow. The pH and conductivity of snow, and the geochemistry of the sulfide sediments were also studied. The occurrence of fine-grained sediments within the catchment was estimated from topographic and geologic maps and by visit to areas were the overburden type could not be determined from the maps.The hydrological conditions had a strong influence on the water quality. During high water flow in spring and autumn the concentrations of ionic components (conductivity) and hydrogen ions in the stream water were high, while the concentrations were much lower during low and intermediate water flow and when the ground was frozen. Extensive leaching of the sulfide-bearing sediments, which had higher contents of S and metals than the fine fraction of till in the study area, resulted in elevated contents of Li, Ni, Zn, Co, B, Al, Mg, U, Sr, Ca, K and Na in the stream water during autumn high water flow. The contents of Cr, Ba, Pb, Si and Fe in stream water were not much influenced by the type of overburden, while V was depleted in drainage waters from the sulfide sediments. Humic substances flocculated in the water in the middle section of the stream, which may have been caused by di- and trivalent metal cations leached in large quantities from the oxidizing sulfide sediments.