Strontium isotopes evolution of groundwater in a floodplain, Rio Icacos, Luquillo Experimental Forest, Puerto Rico

Major element and Sr concentrations and 87Sr/86Sr ratios were measured in groundwater, rainwater and stream samples collected in2004 from Luquillo Experimental Forest (LEF), Puerto Rico. The groundwater was obtained from wells augured across a ~20 m wide floodplain, which lies between a tributary of Rio Iacos and upslope forest (McDowell et al., Biogeochem 18:53, 1992). The Sr isotope data were also obtained on archived samples collected from the same sites in 1989-1991. The purpose of this study was to investigate (a) controls on the evolution of Sr2+ and their variation between 1989-91 and 2004 and (b) the extent to which floodplain weathering governs the 87Sr/86Sr ratio of the stream. The subsurface water at this site flows through a deep layer of coarse sand and the water table has a gradient of ~0.25. A subset of the data is given below; 1989-91 data are weighted averages of the analyzed samples. % Table generated by Excel2LaTeX from sheet 'Sheet1' \begin{tabular}{lllll} \hline Sample & 2004 & 2004 & 1989-91 & 1989-92 & [Sr], ppb & 87Sr/86Sr & [Sr], ppb & 87Sr/86Sr \hline Rainwater & 4.6 & 0.7101 & & Well (I-9, =4.3 m) & 10.1 & 0.71351 & 1.2 & 0.70875 Well (I-2,d =3.0m) & 18.8 & 0.71035 & & Well (I-4, d= 3.0 m) & 18.1 & 0.70879 & 9.4 & 0.70523 Stream (I-0) & 16.8 & 0.7063 & 15.5 & 0.70512 Downstream of I-0 & 21.5 & 0.70536 & & \hline The rainwater sample is highly enriched in Sr and has an 87Sr/86Sr ratio higher than seawater; the latter possibly reflects a contribution from Saharan dust. Extreme gradients in 87Sr/86Sr are present in waters from both 2004 and 1989-91 with the samples becoming less radiogenic towards the stream. This is accompanied by a 2 to 5 fold increase in the Sr content. It is evident that most of the stream water Sr is derived from subsurface water, which is significantly modified within the floodplain. The floodplain soil is derived from alteration of quartz diorite bedrock containing biotite, hornblende, plagioclase and quartz (White et al., GCA 62:209, 1998; Murphy et al. GCA 62: 227, 1998; Turner et al., Chem. Geol.202: 313, 2003). Plots of 87Sr/86Sr with K/Sr and Ca+Na/Mg show intense biotite weathering upslope of I-9 and increasing contributions of Sr from unradiogenic plagioclase in the floodplain. As the flushing rate in the floodplain is fast we infer rapid Sr exchange between these minerals and groundwater. Floodplain contribution to Sr budget of the water is substantial and seems to buffer the 87Sr/86Sr ratio of the stream.