Lithological Control of Stream Chemistry in the Luquillo Mountains, Puerto Rico

Meteoric waters move along pathways in the subsurface that differ as a function of lithology because effects chemical and physical weathering. To explore how this affects stream chemistry, we investigated watersheds around an igneous intrusion Luquillo Mountains (Puerto Rico). We analyzed streams on 1) unmetamorphosed country rock (volcaniclastic sedimentary strata, VC) surrounding intrusion, 2) quartz-diorite (QD), 3) metamorphosed aureole (hornfels-facies volcaniclastics, HF). These lithologies physically chemically but weather under same tropical rain forest conditions. The VC is pervasively fractured while massive QD HF are relatively unfractured. However, fractures during weathering to produce spheroidally-weathered corestones surrounded by cm-thick rindlets increasingly weathered rock. flow through network already-fractured spheroidally QD, only access limited fraction HF, explaining why draining most dilute mountains. This results various thicknesses regolith from thick (VC) moderate (QD) thin or nonexistent (HF). pervasive allow groundwater deeply then return mainstem river (Río Mameyes) at lower elevations. “rock waters” drive concentrations rock-derived solutes (silica, base cations, sulfate, phosphate) higher reaches stream. Water also flows weathering-induced high elevations where rindletted present stacks, water flux dissolves plagioclase hornblende oxidizes biotite. “QD water” not generated Río Icacos watershed, stacks absent, contributions derive feldspar- hornblende-depleted saprolite. chemistry QD-dominated watershed Icacos) thus varies concentrated QD-rock channel heads below steep ridgelines toward more diluted “saprolite downstream. observations emphasize importance fracture patterns dictating flowpaths, development headwater catchments.