Guo, and 3 co-authors, reported in 2010 that the groundwater and sediments were sampled along a groundwater flow path in the shallow aquifer of the Hetao Basin, Inner Mongolia. Along a flow path with a length of about 9 km, groundwater evolves from Ca–HCO3–SO4 through Mg–Ca–HCO3–Cl to Na–Cl–HCO3 from the recharge zone to the discharge zone. Eh and other redox indicators (i.e., dissolved Fe, H2S, U and As concentrations) indicate that, except for those in the recharge zone and immediately to the southeast of a groundwater drainage canal, the groundwater was reducing and exhibited a progressive decrease in redox conditions along the flow path. Rare earth element concentrations exhibited less variation along the flow path, except that Eu concentration generally increases.
Arsenic concentration is positively correlated with Eu concentration, while both of them are negatively correlated with Eh. North American Shale Composite (NASC)-normalized REE patterns are enriched in the HREEs relative to the LREEs for all the groundwater samples. They have:
positive Eu anomalies: Eu/Eu* = EuNASC/(SmNASC × GdNASC)0.5 = 3.42 to 239 and
slightly negative Ce anomalies: Ce/Ce* = CeNASC/(LaNASC × PrNASC)0.5 = 0.39 to 1.05.
Reduction–oxidation, complexation, desorption and re-adsorption alter groundwater REE concentrations and fractionation patterns along the groundwater flow path. The positive Eu anomalies in the groundwater are probably due to preferential mobilization of Eu2+ relative to the trivalent REEs in the reducing condition. The same conditions causing the positive Eu anomalies probably also lead to As mobilization in the shallow aquifer.
The authors concluded that redox conditions change along the flow path. Eh and other redox indicators (i.e., dissolved Fe, H2S, U and As concentrations) indicate that groundwater, in the region of the aquifer proximal to the recharge zone and immediately to the southeast of the groundwater drainage canal, exhibits oxic–suboxic conditions. However, between ∼1.5 and ∼4 km along the flow path, and beyond well 49 (∼5.5 km), groundwater are reducing and exhibit a progressive decrease in redox conditions. Within the region of the aquifer nearest to the recharge zone (i.e., 0–1 km), a variety of oxidation–reduction reactions are possibly important, including aerobic respiration and denitrification. Between ∼1 and ∼2 km along the flow path, and beyond well 49 (∼5.5 km), Fe(III) reduction occurs, whereas the increase in H2S concentration between ∼2 and ∼3.5 km indicates sulfate reduction is important.
With exception of Eu, the REE concentrations initially show small fluctuations along the groundwater flow path until about 3.7 km (i.e., well 48), and keep relatively constant for the remainder of the flow path sampled. Generally, the increase in REE concentration occurs in phase with the decline in redox potential. Europium concentrations remain essentially the same from well 43 to well 45, and then fluctuate with groundwater flow beyond well 45, which are positively correlated with As concentrations. NASC-normalized REE patterns are characterized by HREE enrichment and LREE depletion for all the groundwater samples. The Eu/Eu* ratios of all groundwater are greater than 3, showing groundwater have positive Eu anomalies. The reducing condition is believed to favor Eu enrichment in groundwater.
Changes in REE concentrations and shale-normalized patterns indicate that the REEs are not conservative tracers in shallow groundwater. Along the groundwater flow path, reduction, desorption and complexation account for the elevated groundwater REE concentrations, while carbonate complexation and re-adsorption result in enrichment of the HREEs relative to the LREEs. The major contribution to positive Eu anomalies in groundwater is expected to be preferential mobilization of Eu2+ relative to the trivalent REEs. Since As concentrations vary in phase with Eu concentrations in groundwater, the geochemical controls on positive Eu anomalies imply similar mechanisms for As mobilization in the shallow aquifer in the study area.
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