Groundwater-Surface Water Interactions Groundwater and surface water are intertwined Different types of interactions of groundwater with: –streams and rivers –lakes –wetlands –oceans Focus on groundwater-stream interactions –gaining vs losing –measurements –role of hyporheic zone

Groundwater- Stream Interactions Gaining reach Losing reach Groundwater can discharge to streams (gaining stream) or streams can discharge to groundwater (losing stream). Gaining and losing reaches can occur along the same stream (see example above) Fetter, 2000

Determine gaining/losing stream via equipotentials Gaining Stream: convex equipotential lines zones of groundwater discharge net increase in flow Losing Stream: concave equipotential lines zones of groundwater recharge net loss of flow Freeze and Cheery (1979)

Hyporheic Zone: region between groundwater and surface water Defined as the portion of the groundwater interface in streams where mixture of surface and groundwater is found. Occurs beneath the active channel and within the riparian zone

Key Components of the Hyporheic Zone interface of groundwater and channel water associated gradients in biogeochemical variables (pH, redox, microbial populations, organic content, light, temperature)

Hyporheic zone: ground water habitat of stream ecosystems Stygobromus: subterranean amphipod hyporheic mayfly Original use by Orghidan (1959) as groundwater environment with distinctive biota

Hyporheic zone metabolically active, impacts nutrient cycling, which impacts stream ecology Upwelling waters bring groundwater nutrients to stream channel Controls on upwelling vs. downwelling Streambed morphology Vertical Gradients Dahm and Valett, 1996

dh dL = ' - ' = ' + ' dh Downwelling Upwelling Vertical Hydraulic Gradients measured using piezometers or manometers dL dh dL dh free water surface dL

Temporal variations in gradients can be significant VHG (cm/cm) from Valett (1993) Influence of ET Influence of Floods Time (days) head (m) downwelling 0.8m below stream bed stream upwelling after Lee and Hynes (1977)

Groundwater-stream interactions exert control on solute cycling, biota, and stream hydrology Can be better understood through: delineating gaining (upwelling) and losing (downwelling) reaches through measuring vertical gradients quantifying the upwelling or downwelling discharge (Q) using stream gauging methods mass balance methods to assess the role of the hyporheic zone in impacting solute transformation, retention and/or release Challenge: spatial and temporal variability may be significant!

Case Study 1: Hyporheic influences on Sycamore Creek, AZ (Valett et al., 1994) upstream algal communities (750 mg/m 2 as Chl a) dominated by green algae 100 m downstream algal communities (98 mg/m 2 as Chl a) dominated by bluegreen bacteria

Flash flooding represents a disturbance that reduces Chl to below detection limit and kills 95-99% of all invertebrate fauna From Valett et al. (1994)

Patterns of GW/SW Exchange: Sycamore Creek From Valett et al. (1994) VHG: VHG: ~ 0 VHG: 0 to -1 Dry Stream Bed

after Valett et al. (1994) From Valett et al. (1994)

GW/SW Exchange and Ecosystem Resilience after Valett et al. (1994) From Valett et al. (1994)

Case Study 2: Groundwater-stream interactions in a mine setting Arsenopyrite-bearing schist Produced As 2 O Brinton Arsenic Mine Lottig, 2005 Watson, 1911

Mine Foundation Waste piles 50 m N Waste piles resulting from mining operations lie adjacent to a headwater stream Streamflow Start End Schreiber, unpublished

Waste pile stream Gaining stream Groundwater contributes high concentrations of As to stream Schreiber, unpublished

Is there any retention of arsenic in the HZ as groundwater discharges to the surface? Gross groundwater load Groundwater arsenic concentration Groundwater discharge Hyporheic zone Stream [As] Use a mass-balance approach. First, find the gross groundwater load. B. Brown, unpublished

Now, find the net groundwater load: the difference between loads entering and leaving the study reach Net groundwater load Load entering the study reach Load leaving the study reach Hyporheic zone Stream B. Brown, unpublished

Now, calculate retention: the difference between the gross and net groundwater loads If R HZ = negative, => net release in HZ If R HZ = positive, => net retention in HZ Hyporheic zone Stream B. Brown, unpublished

Using the mass-balance method, whole reach retention of As can be calculated Process: (1)Delineation of hyporheic zone (2)Establishment of the spatial variability of groundwater discharge (3)Characterization of groundwater flowpaths (4)Preliminary assessment of subsurface arsenic concentrations Stream B. Brown, unpublished