Map of Ireland showing all major rivers and catchments in Ireland and Northern Ireland grouped as in the text with simplified peat lands (Lydon and Smith, 2014). Numbers correspond to catchment groups given in Table 2. Original data were obtained from Ireland EPA Geoportal (www.gis.epa.ie) and (www.opendatani.gov.uk/dataset/priorityhabitats_peatland).

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... this paper, we investigate the chemical composition of 21 of the 22 largest rivers by discharge on the island of Ireland (EPA, 2021) based on 109 grab samples (Appendix A, Fig. 2). The rivers in this study are separated into five groups: 1) first-and second-order (Lydon and Smith, 2014). Numbers correspond to catchment groups given in Table 2. Original data were obtained from Ireland EPA Geoportal (www.gis.epa.ie) and (www.opendatani.gov.uk/dataset/priorityhabitats_peatland). streams draining lowland (Atlantic) ...

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... rocks that are covered in most locations by Pleistocene glacial and Holocene post-glacial deposits (Holland and Sanders, 2009). Although we do present data from watersheds containing Late Precambrian to Early Cambrian Dalradian metamorphic rocks and from igneous intrusives, the majority of the watersheds are dominated by the Paleozoic sediments ( Fig. 1; Fig. 2; Table 1). The detailed geology of Ireland (see Holland and Sanders (2009)) will not be repeated here, but instead we discuss the general importance of lithology as it relates directly to the riverine geochemistry throughout the ...

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... shale derived till with a substantial area of granite derived till in the east. These Quaternary sediments also fall along the Northern Ireland-Ireland south border. Limestone derived till dominates the center of Ireland, with intermittent peat, sandstone and shale derived tills also incorporated. Peatlands are most prevalent in northwest Ireland (Fig. 2), along with metamorphic derived till ( Elío et al., 2017). This simplification can be further defined by region, although these are generalizations, and in many areas, bedrock is covered by Holocene peat and the substrate is obscured. Detailed Quaternary geology, bedrock geology, and groundwater aquifer maps can be viewed using the ...

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... a first step to determine solute sources, a Pearson correlation matrix was developed to assess the relationships among dissolved constituents in all the Irish rivers (Fig. S2). The strongest positive correlations were found for TDS-Ca, TDS-HCO 3 and Ca-HCO 3 . Weaker positive correlations were observed for TDS-F, TDS-Mg, HCO 3 -F and HCO 3 -Mg. There were also positive relationships between these pairs: Na-K, Na-Cl, K-Cl, K-SO 4 , Ca-Mg, and SO 4 -Si. No significant negative correlations were identified. ...

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... Shannon at Shannon Pot and Dowra may represent an enrichment of Sr relative to Ca sourced from the dissolution of gypsum from evaporitic beds alternating with shales and limestones observed by West et al. (1968). These authors attribute blocks of gypsum observed on the shore of Lough Allen, between Dowra and Drumshanbo, as sourced from these beds (Fig. 2). Increased SO 4 from these same sample sites in the upper reaches of the Shannon could be coming from the oxidation of reduced sulfur phases in peatland or shales. The combination of higher SO 4 concentrations and lower Ca/Sr ratios may be better explained by the dissolution of other Caand SO 4 -rich sources, such as the gypsum ...

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... the relationship between Si concentrations and alkali metals is shown by correction of precipitation input by subtraction of Cl from Na + K (Fig. 12). In the majority of rivers, the Na and K can be accounted for by precipitation input, as 72 samples have Clcorrected alkali metal concentrations less than zero. More intense weathering of primary cation-rich minerals plot farther to the right, (Stallard and Edmond, 1987;Zhang et al., 2019) including only 37 samples. All three of the ...

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... δ 34 S of SO 4 values are generally higher on the west coast and decrease eastward, but also generally decrease with lower fractions of catchment peatland, though neither of these is a statistically robust relationship (Fig. 2, Table 4). Also summarized in Table 4 are possible sources that were used as end members for our analysis. The most negative δ 34 S values identified originate from the oxidation of biogenically reduced S that has formed pyrite ( Burke et al., 2018;Karim and Veizer, 2000;Torres et al., 2014) and Stallard and Edmond (1987). Values to the left of ...

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... there is little evidence to argue that OWP is the only source of this isotopically depleted S, as the farther inland bog-sequestered SO 4 could have a much more negative δ 34 S value (Novák et al., 2001). In addition, if the oxidation of the reduced S in the bog soils occurs through microbial processes (Pellerin et al., 2019), the SO 4 that is transported from the bog sediments into rivers could have a very different isotopic signal than either what was initially deposited in the bog or what was converted via reduction processes. Given the dearth of S isotope data, we cannot firmly state whether this SO 4 depleted in 34 S is from the more traditional OWP as described by Burke et al. (2018) or from similar oxidation processes occurring in the bog lands. ...

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... rivers and tributaries with mean discharge (m 3 s 1 ), catchment area (km 2 ) and catchment number corresponding to group categories in Fig. 2. Streams draining Connemara bogs (group 1) and Annascaul (group 5(xvii)) did not have discharge data available and are not included. e., molar (Ca + Mg)/HCO 3 > 0.5). This suggests either OWP or SOx gases are promoting the weathering of limestone is contributing to the solute flux (as previously noted, gypsum occurrence is rare). Some ...

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... River Shannon is the longest river with the greatest average flow on the island of Ireland with a catchment area of ~18,000 km 2 , 1/5 th of Ireland's land area, and mean annual discharge of 6.59 × 10 9 m 3 a 1 at Athlone Station (Table 2). Three large lakes, Lough Allen, Lough Ree and Lough Derg, interrupt the flow of the River Shannon (Fig. 2). Land use in the catchment during the 2000s was 1.2 % urban, 9.2 % arable, 65.7 % pasture, and 10.8 % wetland (i.e., primarily bog), with human population density of 34 people km -2 ( Tockner et al., 2009). It is the largest river in Ireland and is used as a drinking water, utility and recreational resource. The length of the river was ...

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... At high discharge in October, the upstream peak occurred at Drumshanbo, followed by a large increase in concentration to a peak downstream at Banagher of 199 μM. The high SO 4 concentrations at Banagher in October were influenced by the Suck and Big Brosna tributaries, which drain subcatchments with ~20 % and ~15 % bog land, respectively (Fig. 2, Fig. 14). Reduced sulfur is oxidized to SO 4 in bogs when the water table is low in summer months, which is then flushed out as a result of water pulses through the system in early autumn ( Daniels et al., 2008), explaining why Banagher exhibited elevated SO 4 concentrations in October. Increased SO 4 concentration is notable specifically at ...