Fractionation of Mg isotopes by clay formation and calcite precipitation in groundwater with long residence times in a sandstone aquifer, Ordos Basin, China

The Control of Groundwater Flow Systems and Geochemical Processes on Groundwater Chemistry: A Case Study in Wushenzhao Basin, NW China

“…This is attributed to the fact that some samples in type 1 waters have been strongly affected by anthropogenic contamination, such as agricultural fertilizer, as indicated by higher mean NO 3 contents in type 1 (range: 13.6-67.8 mg/L; mean: 36.1 mg/L) than type 2 (range: 3.1-31.1 mg/L; mean: 18.5 mg/L) and type 3 (range: 13.6-180.1 mg/L; mean: 2.4 mg/L) (Table S3 and Figure 5). A similar contamination phenomenon caused by human activities is also observed in the regions close to the study area in the Ordos Plateau [20,32].…”

“…Thirty-three samples were gathered from active water wells with The groundwater flow is driven by the wavy topography in the Ordos Plateau, which develops hierarchically nested groundwater flow systems. The local flow system generally develops between groundwater divides and neighboring depressions with a shallow circulation depth of around 200 m; intermediate flow systems occurs between the major divides and the relatively larger lakes with a relatively deep circulation depth; and the regional flow system has a much larger lateral extent and a higher circulation depth (up to 500 m) when compared with the local and intermediate flow systems [14,15,18,20,22].…”

“…The available geochemical investigations of groundwater have been just carried out at a regional scale in the Ordos Plateau [15,17,18], while detailed investigations on the groundwater geochemistry are rather scattered so far, and analyses conducted are limited to the major ions chemistry of groundwater in the Lake Basins, NE Ordos Plateau, such as Dake Lake, Mukai Lake and Hutongchahan (HTCH) lake ( Figure 1b) [14,16,[19][20][21][22]. In addition, the HTCH Lake Basin is chosen as the key study area ( Figure 1c) because it is located in the lowest reaches of the Wushenzhao Basin and might serve as the fingerprint of nested flow systems.…”

Mg isotope fractionation during continental weathering and low temperature carbonation of ultramafic rocks

“…Similarly, surface weathering causes 26 Mg enrichment in the weathering residual, producing Mg-isotope signatures that appear long lasting and robust during subsequent deep burial, potential fluid alteration and partial melting (Shen et al, 2009;Teng et al, 2016;Wang et al, 2017). Concomitant formation of secondary Mg carbonate appears in some cases to balance 26 Mg uptake in secondary silicate phases (Beinlich et al, 2014), while precipitation of 24 Mg enriched carbonate during open system alteration will alter the composition of the weathering runoff, thus contributing to the variability of measured Mg isotope ratios in natural continental waters (Tipper et al, 2006b;Zhang et al, 2018).…”

“…to distinguish changing weathering regimes (Immenhauser et al, 2010;Kasemann et al 2014;Tipper et al, 2006b), to infer the evolution of groundwater (Zhang et al 2018) and to identify the formation of Mg clay rather than Mg-carbonate during subsurface interactions of CO2-bearing fluids with basalt at the CarbFix site (Oelkers et al, 2019). The latter study highlights the importance of understanding the fate of Mg during CO2 sequestration in mafic and ultramafic rocks, as the formation of Mg-clay (as opposed to Mg-carbonate) does not lead to the desired storage of CO2 in carbonate minerals.…”