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Mine waters are a significant point source stressor for aquatic environments, not only due to their acidity and high metal concentrations, but also because of their high electrolyte concentrations. Ion-rich mine waters can disturb the seasonal mixing of lake waters, even leading to permanent stratification, i.e. meromixis. In this study, we investigated two small natural lakes receiving waters from closed Ni-Cu mines. To characterize the present chemical and physical conditions of these two boreal lakes, we collected water samples and in-situ water column measurements seasonally in 2017 and 2018. We modelled the stability of meromixis in the lakes under varying physico-chemical and meteorological conditions with the MATLAB-based open-source model code, MyLake. Chemical analyses and water column measurements show that both lakes are currently meromictic with a chemocline separating the circulating, well-oxygenated upper water from the non-circulating, hypoxic bottom water. The main anion was SO4 in both lakes, while the main cations were Ca, Mg, Na, and K. Elevated concentrations of conservative elements flowing from the mine areas are crucial in maintaining the meromixis. Modelling scenarios suggest that the meromixis would be sustained for several decades even if the external load ceased completely. Lake morphology and sheltered surroundings also seem to contribute to maintaining the meromixis in these lakes. Consequently, our results indicate that small headwaters are sensitive to persistent meromixis even when external loading is mild.

Flocculation of riverine dissolved organic matter to the particulate form in estuaries is an important mechanism for capturing dissolved metals to newly formed organic particles, regulating the metal transport to the sea. The process is particularly relevant for rivers draining boreal acid sulphate soils of western Finland, which 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. This is the first study to investigate dissolved metal (Al, Fe, Mn, Co and Cu) association with flocculating organic particles in the laboratory, by mixing of acidic metal-rich water from acid sulphate soil-impacted rivers and particle-free artificial seawater. Water samples were collected in April 2021 from the Laihianjoki and Sulvanjoki rivers in western Finland. Experiments with an in situ laser diffraction-based particle size distribution sensor and a multiparameter water quality sonde were run to continuously monitor the development of suspended particle pool over the salinity gradient from 0 to 6, corresponding the salinity range observed in these estuarine systems. Flocculator experiments with discrete salinity treatments were carried out to investigate metal behaviour with the collection of flocculated material on glassfibre filters. Filtrate was analysed for coloured dissolved organic matter absorbance and fluorescence for the characterization of potential changes in the organic matter pool during the flocculation process. Retentate on the filter was subjected to persulfate digestion of organic particles and metal oxyhydroxides (pH

To efficiently mitigate bacterial mediated acid and metal discharge from acid sulfate soils, iron-and sulfur-oxidizing microorganisms that catalyze the iron sulfide dissolution should be inactivated. An organic carbon source could further be introduced into the soil to promote the growth of iron- and sulfur-reducing bacteria. In this study, acid sulfate soil was amended with a mobile form of ultrafine calcium carbonate alone or in combination with fractions of peat, sodium acetate, or sodium lactate. The introduction of ultrafine calcium carbonate resulted in a raised pH that appeared to inactivate the acidophiles, but did not reactivate iron- or sulfur-reducing bacteria. The addition of organic matter resulted in higher microbial diversities and retention of metals, although acid-tolerant and acidophilic microbes still dominated. A low abundance of an iron-reducing bacteria was identified in the all treatments with both peat fractions and pure organic carbon compounds. These results indicated that biodegraded peat could be used as an energy source for at least iron-reducing bacteria in the acid sulfate soil at the same time as it retains metals in the soil. These findings are of value for further developing mitigation methods for the sustainable use of acid sulfate soils.

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.

Acid sulfate soils are one of the most environmentally harmful soils existing in nature. This is because theyproduce sulfuric acid and release metals, which may cause several ecological damages. In Finland, the occurrenceof this type of soil in the coastal areas constitutes one of the major environmental problems of the country. Toaddress this problem, it is essential to precisely locate acid sulfate soils. Thus, the creation of occurrence maps forthese soils is required. Nowadays, different machine learning methods can be used following the digital soilmapping approach. The main goal of this study is the evaluation of different supervised machine learningtechniques for acid sulfate soil mapping. The methods analyzed are Random Forest, Gradient Boosting andSupport Vector Machine. We show that Gradient Boosting and Random Forest are suitable methods for theclassification of acid sulfate soils, the resulting probability maps have high precision. However, the accuracy ofthe probability map created with Support Vector Machine is lower because this method overestimates the non-ASsoils occurrences. We also compare these modeled probability maps with the conventionally produced occurrencemap. In general, the modeled maps are more objective and accurate than the conventional maps. Moreover,the mapping process using machine learning techniques is faster and less expensive.

Concern has been raised about the potential formation of acid sulfate soils and associated environmental problems related to peat extraction and, thus, peat, sediment and till characteristics of 15 well drained peat extraction fields were investigated in northern and northwestern Finland. The aim was to identify and characterize the occurrence/abundance of potential acid (hypersulfidic) and actual acid sulfate soil materials as well as metals with regards to their depositional environment (marine/non-marine), black schists and soil material properties. Sulfide-bearing marine sediments were commonly found; the highest contents and thickness of sulfide sediments were found in areas 100 m a.c.s.l.). The highest content of sulfides in sediments was typically found just below the peat layer. The sulfides existed mostly as pyrite (up to 3.5 wt% S) but were occasionally also mixed with more reactive metastable sulfide (Fe:S ratio in the order of 1:1), which coincide with high Fe concentrations, indicating that an abundance of Fe2+ can diminish the rate of pyrite formation. Sediments contained very high amounts of potential acidity, but in-field oxidation of the sediments was very limited. Although the sulfur contents were much lower (max 0.3% S), several of the till samples still became acidic (pH < 4) upon oxidation, and contained, thus, some amounts of potential acidity due to a pore buffering capacity. Consequently, during peat extraction most of the acidity is still retained in sediments limiting environmental consequences and, thus, focus should be on appropriate management to prevent oxidation and leaching after peat extraction has been finished. Locally high Mn and Zn concentrations were found in the sediments in black schist areas, indicating that Zn and Mn have migrated with the ground water from the schists to the sediment layers. In several black schist areas, the peat layer contained high amounts of sulfides with a corresponding high potential acidity as well as elevated As, Pb and Zn. This indicates an upward transport of sulfur and some metals (As, Pb and Zn) through capillary rise and/or plant uptake and/or through lateral water transport from surrounding black schist affected soil material. Air pollution, i.e. atmospheric distribution, was most likely a source for high concentrations of Cd, Hg and Sr concentrations in the peat.

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.

Besides causing acidification, acid sulfate (AS) soils contain large nitrogen (N) stocks and are a potential source of N loading to waters and nitrous oxide (N2O) emissions. We quantified the stocks and flows of N, including crop yields, N leaching, and N2O emissions, in a cultivated AS soil in western Finland. We also investigated whether controlled drainage (CD) and sub-irrigation (CDI) to keep the sulfidic horizons inundated can alleviate N losses. Total N stock at 0–100 cm (19.5 Mg ha−1) was smaller than at 100–200 cm (26.6 Mg ha−1), and the mineral N stock was largest below 170 cm. Annual N leaching (31–91 kg N ha−1) plus N in harvested grain (74–122 kg N ha−1) was 148% (range 118–189%) of N applied in fertilizers (90–125 kg N ha−1) in 2011–2017, suggesting substantial N supply from soil reserves. Annual emissions of N2O measured during 2 years were 8–28 kg N ha−1. The most probable reasons for high N2O emission rates in AS soils are concomitant large mineral N pools with fluctuating redox conditions and low pH in the oxidized subsoil, all favoring formation of N2O in nitrification and denitrification. Although the groundwater level was higher in CD and CDI than in conventional drainage, N load and crop offtake did not differ between the drainage methods, but there were differences in emissions. Nitrogen flows to the atmosphere and drainage water were clearly larger than those in non-AS mineral soils indicating that AS soils are potential hotspots of environmental impacts.

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.

Sediments along the Baltic Sea coast can contain considerable amounts of metal sulfides that if dredged and the spoils deposited such that they are exposed to air, can release high concentrations of acid and toxic metals into recipient water bodies. Two river estuaries in western Finland were dredged from 2013 to 2018 and the dredge spoils were deposited on land previously covered with agricultural limestone to buffer the pH and mitigate acid and metal release. In this study, the geochemistry and 16S rRNA gene amplicon based bacterial communities were investigated over time to explore whether the application of lime prevented a conversion of the dredge spoils into acid producing and metal releasing soil. The pH of the dredge spoils decreased with time indicating metal sulfide oxidation and resulted in elevated sulfate concentrations along with a concomitant release of metals. However, calculations indicated only approximately 5% of the added lime had been dissolved. The bacterial communities decreased in diversity with the lowering of the pH as taxa most similar to extremely acidophilic sulfur, and in some cases iron, oxidizing Acidithiobacillus species became the dominant characterized genus in the deposited dredge spoils as the oxidation front moved deeper. In addition, other taxa characterized as involved in oxidation of iron or sulfur were identified including Gallionella, Sulfuricurvum, and Sulfurimonas. These data suggest there was a rapid conversion of the dredge spoils to severely acidic soil similar to actual acid sulfate soil and that the lime placed on the land prior to deposition of the spoils, and later ploughed into the dry dredge spoils, was insufficient to halt this process. Hence, future dredging and deposition of dredge spoils containing metal sulfides should not only take into account the amount of lime used for buffering but also its grain size and mixing into the soil.

In Finland, acid sulfate (AS) soils are regarded as a serious environmental threat towards the Baltic Sea and watersheds situated in land areas that have emerged from the sea since the last glaciation due to glacial isostasy. The aim of this study is to compare the behavior of coarse-grained AS soil materials to the behavior of fine-grained AS soils and coarse-grained non-AS soils in order to (1) assess the potential environmental threat of coarse-grained AS soils and (2) to assess the need to distinguish coarse-grained AS soils from fine-grained AS soils in future risk assessment of AS soils. The hypotheses are that (1) a coarser grain size enhances the rate of oxidation and pH decrease due to larger inherited pore size, enabling better initial aeration and less efficient buffering processes due to lower particle surface areas and different mineralogy (more weathering resistant quartz and feldspars), and that (2) the leaching of acid generating materials and elements from coarse grained AS soils is significantly lower as compared to fine-grained AS soils but significantly higher as compared to coarse grained non-AS soils. The study used an incubation based “let the soil speak for itself”-approach, which means that parameters such as pH, acidity, amount of elements leached, and the electric conductivity of the leachates were measured on arrays of sample aliquots before, during, and after a 16 week incubation (oxidation) period. Even though the coarse-grained AS soil materials contained one order of magnitude less sulfide than the fine-grained AS soil materials, a S mass fraction as low as 0.01%, the pH dropped well below 4.0 upon oxidation, thus being classified as AS soils. The amount of acid generating materials and elements leached from coarse-grained AS soil materials were at least one order of magnitude less, as compared to the fine-grained AS soil materials, except for Fe, which leached in similar or greater quantities from coarse-grained AS soil materials. The differences in the leaching of acid generating materials and elements suggest it would be beneficial to divide coarse-grained and fine-grained AS soils into separate subgroups for risk management purposes.

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.

Sulfide-bearing anoxic sediments are found in coastal regions around the world including Australia and the Baltic. Upon lowering of the groundwater by drainage, they are oxidized and form acid sulf...

The purpose of this study was to investigate the influence of soil geochemistry on the concentrations of Ca, K, Mg, P, Co, Ni, Zn, Mn, Cu, and Fe in cabbage (Brassica oleracea L. var. capitata) grown on acid sulfate (AS) soils in Western Finland. A total of 11 topsoil (0–20 cm) and corresponding cabbage samples and three whole-soil profiles (≈ 0–260 cm) were collected on three agricultural fields. The concentrations of Co and Zn in cabbage were correlated with the NH4Ac-extractable (easily available) concentrations in the topsoil, indicating that the uptake of these elements in cabbage is largely governed by soil geochemistry. Yet, the concentrations of Co and Zn in cabbage were not in general elevated relative to that of Finnish average values, although some AS soils showed enriched concentrations of these metals in the soil and cabbage. Significant geochemical differences (e.g., oxidation depth, organic-matter and S content, pH) were observed among the studied AS soils, while, on the other hand, the concentrations of Ca, K, Mg, P, Ni, Mn, Cu, and Fe in cabbage were relatively similar. The hydroxylamine-extractable concentrations of these elements in the topsoil were not correlated to those in cabbage, suggesting that uptake is not governed by the oxide-bound fraction of these elements in the soil. Similarly, the easily available concentrations of Ca, P, Ni, Mn, Cu, and Fe in the topsoil were not correlated to those in cabbage, indicating that uptake is independent of the easily available concentrations in the soil. Hence, it is suggested that cabbage can regulate and thus optimize its concentrations of Ca, P, Ni, Mn, Cu, and Fe. Oxidation depth affected neither the easily available concentrations of Co, Ni, Zn, and Mn in the topsoil nor the concentrations in cabbage. However, the subsoil with a lower oxidation depth, which is to a smaller extent affected by leaching, may partly be enriched in these metals. Nevertheless, these showed no increased concentrations in cabbage. Based on these findings, it is suggested that the large amounts of metals mobilized in AS soils are easily lost to drains, subsequently contaminating nearby waterways and estuaries whereas they are only partly enriched in cabbage and other previously studied crops (oat).

The aim of the study was to investigate the impact of soil chemistry on the concentrations of Co, Ni, Zn, Mn, Cu and Fe in oats (Avena sativa L. cv. Fiia) grown on Finnish acid sulphate (AS) soils with varying geochemical characteristics. Twenty two soil profiles, which were sampled to a depth of 1 m (five 20 cm section splits), and 26 composite oat grain samples were collected on a total of five fields. The concentrations of Co, Ni, Zn and Mn in the grains were correlated with the NH4Ac-EDTA-extractable concentrations in the soils. However, as these four chalcophilic metals are in general easily lost to drains and not retained as a large pool in the soil in easily-extractable form, also the concentrations in the oats were not in general elevated as compared with average values on other soils. On one of the fields, however, the Co and Ni concentrations in the soil, and thus also in the oats, were clearly elevated. Copper and Fe displayed no correlation between the soil and oat concentrations, indicating that the plant-uptake mechanisms are much more important than variations in geochemistry. It was suggested that the NH4Ac-EDTA solution was not efficient in extracting Fe and Cu, which shows that these metals are bound in relatively immobile oxyhydroxides.

Soils containing an approximately equal mixture of metastable iron sulfides and pyrite occur in the boreal Ostrobothnian coastal region of Finland, termed ‘potential acid sulfate soil materials’. If the iron sulfides are exposed to air, oxidation reactions result in acid and metal release to the environment that can cause severe damage. Despite that acidophilic microorganisms catalyze acid and metal release from sulfide minerals, the microbiology of acid sulfate soil (ASS) materials has been neglected. The molecular phylogeny of a depth profile through the plough and oxidized ASS layers identified several known acidophilic microorganisms and environmental clones previously identified from acid- and metal-contaminated environments. In addition, several of the 16S rRNA gene sequences were more similar to sequences previously identified from cold environments. Leaching of the metastable iron sulfides and pyrite with an ASS microbial enrichment culture incubated at low pH accelerated metal release, suggesting microorganisms capable of catalyzing metal sulfide oxidation were present. The 16S rRNA gene analysis showed the presence of species similar to Acidocella sp. and other clones identified from acid mine environments. These data support that acid and metal release from ASSs was catalyzed by indigenous microorganisms adapted to low pH.

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.

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.

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.