Abstract:Various complementary techniques were used to investigate the stormflow generating processes in a small headwater catchment in northeastern Puerto Rico. Over 100 samples were taken of soil matrix water, macropore flow, streamflow and precipitation, mainly during two storms of contrasting magnitude, for the analysis of calcium, magnesium, silicon, potassium, sodium and chloride. These were combined with hydrometric information on streamflow, return flow, precipitation, throughfall and soil moisture to distinguish water following different flow paths. Geo-electric sounding was used to survey the subsurface structure of the catchment, revealing a weathering front that coincided with the elevation of the stream channel instead of running parallel to surface topography. The hydrometric data were used in combination with soil physical data, a one-dimensional soil water model (VAMPS) and a three-component chemical mass-balance mixing model to describe the stormflow response of the catchment. It is inferred that most stormflow travelled through macropores in the top 20 cm of the soil profile. During a large event, saturation overland flow also accounted for a considerable portion of the stormflow, although it was not possible to quantify the associated volume fully. Although the mass-balance mixing model approach gave valuable information about the various flow paths within the catchment, it was not possible to distill the full picture from the model alone; additional hydrometric and soil physical evidence was needed to aid in the interpretation of the model results.

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Geoelectrical and electromagnetic (time and frequency domains) hydrogeophysical methods were applied and jointly interpreted together with auxiliary information such as regional piezometric maps, borehole lithological logs and offshore data. The objective was to retrieve the structure and geometry of the AlbufeiraRibeira de Quarteira coastal aquifer system (Algarve, Portugal) and to upgrade the current hydrogeological conceptual model. The results allowed for the detection of the freshwater-saltwater interface along the coastline and identification of the water-bearing layers and aquitards and their hydraulic relationships. An explanation for the location of the inter-and subtidal fresh groundwater discharge is also presented and a new modeling unit is proposed for groundwater flow modeling. Limitations of the used hydrogeophysical methods are indicated and recommendations are made for follow-up studies.

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