Abstract

Streams draining watersheds of the two dominant lithologies
(quartz diorite and volcaniclastic rock) in the Luquillo
Experimental Forest of eastern Puerto Rico have very high
fluxes of bedrock weathering products. The Río Blanco quartz
diorite in the Icacos watershed and the Fajardo volcaniclastic
rocks in the Mameyes watershed have some of the fastest
documented rates of chemical weathering of siliceous rocks
in the world. Rapid weathering produces thick, highly leached
saprolites in both watersheds that lie just below the soil and
largely isolate subsurface biogeochemical and hydrologic
processes from those in the soil. The quartz diorite bedrock
in the Icacos watershed weathers spheroidally, leaving large,
relatively unweathered corestones that are enveloped by
slightly weathered rock layers called rindlets. The rindlets
wrap around the corestones like an onionskin.
Within the corestones, biotite oxidation is thought to
induce the spheroidal fracturing that leads to development
of rindlets; plagioclase in the rindlets dissolves, creating
additional pore spaces. Near the rindlet-saprolite interface,
the remaining plagioclase dissolves, hornblende dissolves to
completion, and precipitation of kaolinite, gibbsite, and goethite
becomes pervasive. In the saprolite, biotite weathers to
kaolinite and quartz begins to dissolve. In the soil layer, both
quartz and kaolinite dissolve. The volcaniclastic bedrock of
the Mameyes watershed weathers even faster than the quartz
diorite bedrock of the Icacos watershed, leaving thicker saprolites
that are devoid of all primary minerals except quartz. The
quartz content of volcaniclastic bedrock may help to control
watershed geomorphology; high-quartz rocks form thick saprolites
that blanket ridges.
Hydrologic flow paths within the weathering profiles
vary with total fluid flux, and they influence the chemistry of
streams. Under low-flow conditions, the Río Icacos and its
tributaries are fed by rainfall and by groundwater from the
fracture zones; during storm events, intense rainfall rapidly
raises stream levels and water is flushed through the soil as
shallow flow. As a result, weathering constituents that shed
into streamwaters are dominated by rindlet-zone weathering
processes during base flow and by soil weathering processes
during stormflow. The upper reaches of the Mameyes watershed
are characterized by regolith more than 35 meters thick
in places that contains highly fractured rock embedded in its
matrix. Weathering contributions to stream chemistry at base
flow are predicted to be more spatially variable in the Mameyes
watershed than in the Icacos watershed owing to the
more complex subsurface weathering profile of the volcaniclastic
bedrocks of the Mameyes watershed.

Citation

Buss H.L., and White A.F. (2012): Water quality and landscape processes of four watersheds in eastern Puerto Rico. USGS.

This Paper/Book acknowledges NSF CZO grant support.