Trace element patterns in Dutch coastal dunes after 50 years of artificial recharge with Rhine River water

Trace elements (TEs) are important in water quality monitoring of managed aquifer recharge (MAR) systems, because they need to be tested against maximum permissible concentrations (MPCs), and some can be used as a tracer of either infiltration water, pollution or geochemical processes. The behavior of 55 dissolved trace elements and 20 major constituents is shown for an 800 m long, 35 m deep transect with slow groundwater flow (similar to 0.01-0.1 m/day) and a nearby 100 m long 20 m deep transect with fast groundwater flow (0.3-2 m/day), both between a recharge and recovery canal, in the coastal dune aquifer system of the Netherlands. This study demonstrates how TEs behaved in a pH 7-8 artificial recharge system in coastal sediments, where pretreated Rhine River water has been infiltrated in the period 1957-2007. The spatial and age distribution of the infiltrated Rhine water was mapped using a combination of environmental tracers (Cl/Br, O-18, H-3, Cl and SO4), allowing the comparison between young and old Rhine water, and between Rhine and dune groundwater. It is revealed which TEs can be used in specific cases as additional tracers (B, F, Mo), which TEs showed clear redox dependent behavior (As, Mo, U) or clear sorptive behavior (B, Ba, Co, Cs, Cu, F, Li, Mo, Ni, Rb, Sb, Sr, U, W), which TEs received significant geogenic inputs (As, Sr, U, W), which 'classical' TEs need much less monitoring compared to the 1970s and 1980s (B, Ba, Be, Cd, Co, Ni, Hg, Pb, Zn), and which TEs can normally be neglected (rare earth elements, precious metals).