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Due to discharge from acid sulfate (a.s.) soils, watercourses and coastal areas in the Gulf of Bothnia are periodically heavily acidified with high concentrations of potentially toxic metals. Data on water quality from 2005 to 2014 in an embanked lake, an estuary of four rivers in western Finland, showed repeated events with acidic water (pH Lota lotaL.) was shown to be sensitive to acidity during the wintertime spawning migration and spawning. Bearing in mind the importance of estuaries of the northern Baltic Sea as spawning and nursery areas of fish, the reoccurring failure in the reproduction of fish may cause a more serious threat for the lake and adjacent coastal fish stocks than the spectacular, but less frequent, mass kills of adult fish. This demonstrates the close relationship between climate, hydrology, water geochemistry and the aquatic coastal ecosystem in areas affected by a.s. soils. As the current forecast of climate chance indicates warmer winters with more continuous runoff, the effects can become even more prominent. This study also shows that the annual larvae abundance of burbot may be used as a bioindicator and an instrument for the fisheries for obtaining more comprehensive knowledge of the ecological effects of acidic metal discharge from a.s. soils.

Rivers draining the acid sulfate soils of western Finland are known to deliver large amounts of trace metals with detrimental environmental consequences to the recipient estuaries in the eastern Gulf of Bothnia, northern Baltic Sea. However, the distribution of these metals in the coastal sea area and the relevant metal transport mechanisms have been less studied. This study investigates the spatial and temporal distribution of metals in sediments at nine sites in the Kvarken Archipelago, which is the recipient system of Laihianjoki and Sulvanjoki rivers that are impacted by acid sulfate soils. The contents of Cd, Co, Cu, La, Mn, Ni, and Zn increased in the cores during the 1960s and 1970s as a consequence of intensive artificial drainage of the acid sulfate soil landscape. Metal deposition has remained at high levels since the 1980s. The metal enrichment in sea floor sediments is currently visible at least 25 km seaward from the river mouths. Comparison with sediment quality guidelines shows that the metal contents are very likely to cause detrimental effects on marine biota more than 12 km out from the river mouths. The dynamic sedimentary environment of the shallow archipelago makes these sediments potential future sources of metals to the ecosystem. Finally, the strong association of metals and nutrients in the same sediment grain size class of 2–6 µm suggests that the transformation of dissolved organic matter and metals to metal–organic aggregates at the river mouths is the key mechanism of seaward trace metal transport, in addition to co-precipitation with Mn oxyhydroxides identified in previous studies. The large share of terrestrial organic carbon in the total organic C in these sediments (interquartile range – 39 %–48 %) highlights the importance of riverine organic matter supply. These findings are important for the estimation of environmental risks and the management of biologically sensitive coastal sea ecosystems.

Some of the most economically valued soils for agricultural use are naturally occurring sulfide rich sediments. However, formation of acid sulfate soils with sulfuric materials (pH ≤ 4) can occur when sulfidic materials are exposed to air, which can then result in mobilisation of large amounts of acid and metals into nearby water bodies. In this study, controlled drainage, subsurface irrigation and hydrochemical precision treatments are combined to reduce acidic discharges on a novel project field in western Finland. The PRECIKEM project field consists of nine identical hydrologically isolated 1 ha subfields. Each field had a drainage system consisting of three subsurface drainage pipes (c. 1.3 m deep), a collector pipe, and a control well enabling manual groundwater table management. Utilising such drainage installations already common on farmlands, suspensions of fine-grained (d50 = 2.5 μm) calcium carbonate and/or calcium hydroxide were pumped in to control wells in order to be distributed in to subsoils with sulfuric materials via drainage networks with the aim to: (1) neutralise acidity, (2) inhibit microbially mediated sulfide oxidation and (3) immobilise metals. The discharge waters from the fields were monitored during the project period 2012–2016. As is typical for acid sulfate soils with sulfuric materials, the discharge waters from the reference fields (n = 3) that had been treated with water only, had very low pH values (≤4) and the acidity and concentrations of several metals were up to two magnitudes higher than the average in Finnish stream waters. Excavation of selected treated fields revealed the calcium carbonate to have formed a neutralising coating on the surfaces of hydrologically active macropores in the soil matrix near the subsurface drainage pipes. This effectively resulted in a long-term (1–4 years) situation of raised pH, lower acidity and lower concentrations of several acid sensitive metals, most prominently a significant decrease (>90%) in Al concentrations. Fe concentrations in discharge waters were subsequently decreased as the predominance of Fe shifted toward the schwertmannite and iron oxides stability phases due to changes in pH/redox conditions. The methods presented in this work showed favourable steps toward environmentally sustainable agriculture and improving the chemical and ecological status of acid sulfate soil affected coastal waters.

Leaching large amounts of acidity and metals into recipient watercourses and estuaries, acid sulfate (a.s.) soilsconstitute a substantial environmental issue worldwide. Mapping of these soils enables measures to be takento prevent pollution in high risk areas. In Denmark, legislation prohibits drainage of areas classified as potentiala.s. soilswithout prior permission fromenvironmental authorities. Themapping of these soils was first conductedin the 1980’s.Wetlands, inwhich Danish potential a.s. soils mostly occur,were targeted and the soilswere surveyedthrough conventional mapping. In this study, a probability map for potential a.s. soil occurrence wasconstructed for thewetlands located in Jutland, Denmark (c. 6500 km2), using the digital soilmapping (DSM) approach.Among the variety of available DSM techniques, artificial neural networks (ANNs) were selected. Morethan 8000 existing soil observations and 16 environmental variables, including geology, landscape type, landuse and terrain parameters,were available as input datawithin themodeling. Predictionmodels based on variousnetwork topologieswere assessed for different selections of soil observations and combinations of environmentalvariables. The overall prediction accuracy based on a 30% hold-back validation data reached 70%. Furthermore,the conventional map indicated 32% of the study area (c. 2100 km2) as having a high frequency for potentiala.s. soils while the digital map displayed about 46% (c. 3000 km2) as high probability areas for potential a.s.soil occurrence. ANNs, thus, demonstrated promising predictive classification abilities for themapping of potentiala.s. soils on a large extent.

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The estuaries of the Northern Baltic Sea (Gulf of Bothnia) receive an abundance of diagenetically reactivecatchment-derived Fe, which is to a large degree complexed with organic matter or present a ...

In Finland, poor water quality and associated ecological damage in the coastal streams related to land use on acid sulfate (a.s.) soils has been drawing a considerable amount of attention since the 1950’s. These soils originate from sulfide-bearing marine sediments mostly occurring in the coastal areas located below the highest shoreline of the former Litorina Sea. Of the many previous studies carried out on soil or water data, quite few gathered both and their geographic extent was relatively limited. This study aimed at assessing a.s. soil probability maps using two chemical indicators measured in the recipient waters (i.e. sulfate content and sulfate/chloride ratio) for 24 catchments along the Finnish coast. All the available data was compiled for these catchments, which were surveyed using different methods (i.e. conventional mapping and two spatial modeling techniques: fuzzy logic and artificial neural networks). High sulfate contents and sulfate/ chloride ratios measured in these rivers were controlled by a.s. soils in the corresponding catchments. The extent of the most probable areas for a.s. soils in the surveyed catchments correlated with the two chemical indicators measured in the recipient waters, suggesting that the probability maps created with different methods are reliable and comparable. The use of a.s. soil related chemical indicators in water, thus, constitutes a complementary, independent and straightforward tool to assess a.s. soil probability maps.

This study focuses on attenuation of rare earth elements (REE) when a boreal creek, acidified and loaded with REE and other metals as a result of wetland drainage, empties into a brackish-water est ...

Dissolved ( 0.45 μm) size fractions of 30 elements were determined for four rivers (Sirppujoki, Laajoki, Mynajoki and Paimionjoki), including 12 low-order inflow streams, largely affected by soil erosion and acidity in SW Finland. In addition, geochemical modelling was used to predict the formation of free ions and complexes in these rivers. Total metal concentrations were relatively high but most of the elements occurred mainly in a colloidal or particulate form and even elements expected to be very soluble occurred to a large extent in colloidal form. According to geochemical modelling these patterns could be explained by in-stream metal complexation/adsorption only to a limited extent. Instead there were strong indications that the high metal concentrations and dominant solid fractions were largely caused by erosion of metal bearing phyllosilicates. A strong influence of acid sulphate (AS) soils, known to exist in the catchment, could be clearly distinguished in Sirppujoki river as it had very high concentrations of dissolved metals, while in the two nearby rivers (Laajoki and Mynajoki) the influence of AS soils was largely masked by eroded phyllosilicates. In Paimionjoki river the colloidal and particulate fractions dominated very strongly, indicating that total metal concentrations are almost solely controlled by erosion of phyllosilicates. Consequently, rivers draining clay plains sensitive to erosion, like those in SW Finland, have generally high “background” metal concentrations due to erosion of relatively non-toxic colloidal/particulate phyllosilicates. Thus, relying on only semi-dissolved (

Due to acidity and metals from acid sulfate soils (a.s. soils), many watercourses in midwestern Finland have since 1960s regularly experienced short but intensive periods of poor water quality during high water flow. This has led to occasional massive fish kills and a significant decline in fish populations. However, many watercourses in northern Europe are often also characterized by low pH caused by naturally occurring organic acids, non-carbonate bedrock and previously a large input of acid rain. Therefore the role of acid sulfate soils has not always been properly addressed. This study was done on Larsmo and Oja Lake, a freshwater reservoir embanked from the alkaline Bothnian Bay in the 1960s. Since that, events of severe acidity and fish kills have been documented on several occasions. Severe but heterogeneous a.s. soils with relatively coarse grain sizes were found in the catchment. These soils were found to leach metals typical for a.s. soils such as Al, Cd, Co, Ni and Zn to recipient streams, but in contrast to other a.s. soils studied in Finland, some of them even seem to be leaching large quantities of Fe. Patterns of acidity, sulfate, sulfur isotopes, organic carbon and metals in rivers, streams and the lake revealed that (1) a.s. soils in the catchment are by far the most important source of acidity and metals in the lake i.e. the main reason for fish kills, (2) the load from discharge of organic (humic) acids and dredging operations is insignificant on the lake as a whole, (3) young near-field sediments comprising only 7% of the whole catchment accounted for roughly 20–30% of the total acidic load, (4) most of the acid/metal load entered the lake system from the south, and (5) by regulating outlets to the sea it seems to be possible to prevent this load from dispersing into the northern part of the lake system.

Large areas of acid sulfate (AS) soils are located along the coastal plains of Finland, and previous studies have shown that after reclamation they release extreme quantities of metals to watercourses in mid-western and northern Finland. In this study on streams of south-western Finland, where little information about AS soils is available, these soils were found to exhibit the same pattern of elevated metal- and sulfate concentrations as in the notorious AS soil landscapes of mid-western Finland. Meteorologically/hydrologically driven temporal variations of these elements were great in the most affected streams. There were also significant positive implications regarding future environmental work; AS soils in the highlighted region were found to cause sudden temporal influxes of acidic water only in the most affected streams, indicating that the overburden and soils of the area discharge well buffered water. Moreover, it was indicated that the high (less toxic) metal concentrations are largely caused by erosion of suspended phyllosilicates (

Water samples ( n = 354) from a small catchment (7.4 km 2 ) covered by acid sulphate soils (pH 2 or less), the water quality was relatively good in all seasons. Above base flow conditions, the impact of acid sulphate soils tended to slightly increase with increasing runoff and precipitation, especially in early summer, but not in late summer. No significant signs of dilution during flood conditions (up to 100 L/s km 2 ) were found and neither were there any correlations to rising or falling limbs. The severity of individual summer droughts, which in theory should increase the oxidation of S and acidity in the soils, had little or no impact on the water quality in subsequent autumn and spring. On the other hand, there was a remarkable long-term increase in EC and a corresponding decrease in pH (starting in 1995) after a suite of several very dry summers. After that the water quality did not improve even if the dry summers were followed by some wet summers. This indicates that the temporary pool of readily leachable acidity in the soils is fairly large. Moreover, it indicates that the potential shift towards more extreme global weather conditions (with more severe dry spells) may have significant impacts on the water quality in midwestern Finland, a region that is heavily affected by acidity and metals from acid sulphate soils.

This study examines the spatial and temporal distribution patterns of arsenic (As) in solid and aqueous materials along the mixing zone of an estuary, located in the south-eastern part of the Bothnian Bay and fed by a creek running through an acid sulfate (AS) soil landscape. The concentrations of As in solution form (

The estuarine behaviour of the metal load leaching from acid sulphate (AS) soils was studied in a selected river system (the Vora 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.

The magnitude of the diffuse leakage of metals (Al, As, Cd, Co, Cr, Cu, Fe, Mn, Ni, Pb, V, and Zn) from Finnish acid sulfate soils (AS soils) into streams was assessed and then compared to the metal discharges in effluent from Finnish industry. The diffuse leakage was calculated by using median metal concentrations for a total of 30 ditches draining AS soils and the mean annual runoff from such soils. In comparison to the present-day industrial discharges, AS soils are a massive supplier of Al, Cd, Co, Mn, Ni, and Zn to the aquatic environment. Also Cu exists abundantly in runoff from such soils, while Pb, Cr, Fe, As, and V in general do not. Since the AS soils are anthropogenic (developed as a result of deep ditching for farming purposes), the comparison with industry is justified and shows that there is an urgent need to take measures against the extensive metal leakage from these soils. It is likely that high quantities of metals are leached, in a similar manner, from corresponding soils distributed in many coastal plains worldwide.

Reclaiming of Holocene sulphide-bearing sediments, widespread in the coastal areas of Finland, has enabled oxidation of sulphides to a depth of 1–3 m and the subsequent development of acid sulphate soils (pH < 4). This work is concerned with spatial hydrogeochemical patterns, remediation measures and the juridical obligation to improve water quality in one such area, i.e. the Rintala plain (23 km2) in mid-western Finland. Streams draining acid sulphate soils in Rintala are more acid (pH ~ 4 and acidity ~ 4 mmol l-1) and carry significantly higher concentrations of SO4 2-, Al, Ca, Cd, Co, Cu, F, Mn, Ni, Pb, Se, Sr and Zn than those draining forest and rural areas in the vicinity of the Rintala plain and organic-rich soils located on the plain. The juridical obligation to improve the water quality is inappropriate as it does not consider the main reason for the poor water quality, i.e. drainage by subsurface drainage pipes, and because of the equality principle (other acid sulphate soil areas have just as poor water quality but do not have such an obligation). Groundwater management, i.e. keeping the groundwater level as high as possible, is recommended as the best management practice.