Fase de desarrollo

In the past decade research has begun to focus on the delineation of stream hyporheic zones and the analysis of element transformations within this subsurface environment. Exchanges of stream and groundwater in the hyporheic zone influence stream chemistry as a result of storage and retention as

well as biogeochemical processes (Valett et al., 1994). The hyporheic zone may be a sink or source of

solutes to the stream depending on the relative importance of processes which immobilise or generate

nutrients in the sediments (Triskaet al., 1989a).

At a catchment scale, the lateral and vertical extent of hyporheic zones is controlled by landscape properties such as geological lithology, groundwater magnitude and seasonality, hillslope and channel

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gradients and annual precipitation (Valettet al., 1996). At the individual stream reach scale the size of

the hyporheic zone is determined by geomorphological features of the surface channel such as permeability and variations in stream bed topography which influence stream water slope. Features such as boulders, gravel bars and riffle-pool sequences produce rapid changes in elevation which force streamwater into the subsurface producing localised flow paths in the sediments (Thibodeaux and Boyle, 1987; Harvey and Bencala, 1993).

The size of the hyporheic zone and the extent of interaction between stream water and groundwater may vary in a downstream direction from headwaters to large rivers (Fig. 2). Small perennial headwater streams often have small hyporheic zones on the scale of centimetres because of large groundwater fluxes (White, 1993). The hyporheic zone associated with pool-riffle sequences in mid- order streams can have a vertical and lateral extent on the scale of metres. Tracers injected into a third order gravel bed stream in northern California revealed a high percent of stream water at lateral

distances of up to 10 m from the channel (Triska et al., 1993). Large scale hydrological exchanges of

surface and groundwater extending over distances of kilometres have been described for large river floodplains (Stanford and Ward, 1993). Recently, conceptual models have been proposed which suggest that the importance of the hyporheic zone to stream chemistry is influenced by the proportion of stream water passing through this zone, as well as by water residence time and the rates of

biogeochemical processes within the hyporheic zone (Findlay, 1995; Valettet al., 1996).

Figure 2. The hypothetical representation of trends in the size of the hyporheic zone, from headwaters to large rivers.

Several studies have shown the occurrence of nitrification and denitrification in stream hyporheic

zones during summer base flows (Duff and Triska, 1990; Triska et al., 1993; Joneset al., 1995; Valett

et al., 1996). Nitrogen injection experiments indicated that ammonium in groundwater entering the

hyporheic zone of a northern California stream was oxidised to nitrate in aerobic areas of the zone, whereas nitrate transported to low dissolved oxygen regions was either denitrified or reduced to

ammonium (Triskaet al., 1993). The net effect of various N cycling processes in the hyporheic zone of

this stream was an overall increase in nitrate concentration of stream water (Triska et al., 1989b).

Nitrate concentrations in stream water flowing into the hyporheic zone of a desert stream were also

elevated by nitrification (Joneset al., 1995). Nitrate rich subsurface water was an important source of N

for stream algae as the water re-entered the stream (Valett et al., 1994). Analysis of subsurface flow

few metres of the bar from 2-4 mg L-1 _{in the stream to <1 mg L}-1 _{in the bar (Pinay et al., 1994).} Measurements of in situ denitrification indicated high rates at the upstream end of the bar in areas of organic rich silt and a sharp decline in a downstream direction where sediments were sandy.

Little information is available on P transformations in stream hyporheic zones. Several studies noted

that SRP concentrations were often higher in the hyporheic zone than in surface water (Valett et al.,

1990; Hendricks and White, 1995). The hyporheic zone may be an important source of SRP to streams during periods of surface water nutrient depletion. In a riffle-pool sequence of a third order sand bottom river in northern Michigan, SRP was significantly enriched relative to stream water particularly beneath the downstream end of the riffle where upwelling occurred (Hendricks and

White, 1995). White et al. (1992) noted that macrophytes were often located in these upwelling areas

where SRP was enriched.

Despite these recent studies of particular stream hyporheic zones, knowledge of the role of this subsurface environment as a nutrient buffer zone is limited. Most research has focused on streams with low nutrient loadings in forest and desert landscapes. In these streams the hyporheic zone is frequently a nutrient source for surface waters rather than a sink. It is possible that the hyporheic zone of eutrophic streams may remove nutrients from stream water. However, research on stream denitrification and P adsorption suggest that these processes occur mainly near the sediment-water interface. Consequently, considerable portions of the hyporheic zone may not be involved in the depletion of nutrients in stream water.

Previous research has focused on how stream water is modified by interaction with the hyporheic zone. This stream centred perspective has resulted in an absence of research on the regulation of groundwater nutrient fluxes within this zone. In landscapes where shallow impermeable layers are absent, large groundwater inputs from adjacent uplands may flow at depth beneath stream riparian

zones and discharge directly through the stream bed (Phillips et al., 1993; Bohlke and Denver, 1995;

Hill, 1996). Little information is available on the capacity of stream hyporheic zones to buffer large

nitrate fluxes in groundwater inputs. Robertson et al. (1991) noted some temporal variability in the

removal of nitrate in septic tank effluent that flowed upward through the bed of a small Ontario

stream. Although NO3-N concentrations generally declined from 20 mg L-1to <0.5 mg L-1 in the last

metre of the flow path, in July concentrations rose briefly to 13 mg L-1 _{at the bed surface before}

returning to <0.5 mg L-1_{. In a Maryland agricultural catchment, nitrate contaminated groundwater}

flowed at depth in a thick sand aquifer beneath the riparian zone and discharged upward through the stream bed (Bohlke and Denver, 1995). Although no detailed data are available for the hyporheic zone

of this stream, the high stream base flow NO3-N concentration of 7-10 mg L-1suggests that the stream

bed was not an effective nitrate buffer.