Hydrologic interactions between streams and their subsurface hyporheic zones

CENTRAL QUESTION What is the hyporheic zone? It derives from the Greek meaning under (hypo) the current or flow (rheo).

The hyporheic zone is a portion of the groundwater interface in streams where a mixture of surface water & groundwater can be found (Bencala 1993)

The hyporheic zone In the bed and banks of streams, water and solutes can exchange in both directions across the streambed. This process, termed hyporheic exchange, creates subsurface environments that have variable proportions of water from ground water and surface water. Figure from Winter et al. 2000

The hyporheic zone The bi-directional exchange of water mixes water & solutes from surface and subsurface environments. An empirical perspective from hydrologists recognizes a hyporheic zone as the subsurface zone receiving at least 10% of water by input from the stream (>10% channel water). Definition after Triska et al Figure from Winter et al. 2000

Some important concepts Length and timescale of the interaction help to distinguish hyporheic exchange from the much larger (and longer term) channel and groundwater interactions Figure from Winter et al. 2000

Some important concepts Hyporheic flowpaths leave and return to the stream many times within a single study reach; the exchange of surface water back and forth between the active channel and the subsurface is rapid. Within several kilometers, stream water in relatively small channels is often completely exchanged with the porewater of the hyporheic zone This repeatedly brings stream water into close contact with geochemically and microbially active sediment. Figure from Harvey & Wagner 2000

Steep concentration gradients in dissolved constituents exist at the groundwater/surface water interface, due to mixing of chemically- different waters and chemical transformations Chemical gradients at the interface

Hydrologic exchange regulates sources of oxygen, carbon, & nutrients to organisms and plants This directly controls trophic structure and primary productivity in streams Zonation of metabolic activity in hyporheic zone

Hydrologic fluxes potentially can enhance ecosystem production on both sides of the groundwater/surface water interface, by supplying solutes that are needed in biological metabolism.

hyporheic zone is important to aquatic biota The term hyporheic zone was originally used by Orghidan (1959), who described the interface as a new groundwater environment containing distinctive biota. Higher than expected abundances of aquatic insects were found in sediments where concentrations of oxygen were high.

hyporheic zone can be a source of nutrients: example from Sycamore Creek, Alaska In a study of the ecology of Sycamore Creek near Phoenix Arizona, hydrologic exchange fosters recovery of desert stream ecosystems following storms, by stimulating recycling of the nutrients from buried organic matter.

hyporheic zone at Sycamore Creek, Arizona Photo courtesy of Stuart Fisher, Arizona State University

Following a storm, oxygen advection into hyporheic zone stimulates aerobic decomposition of buried algae. Released nutrients are delivered to the surface channel by return flows from hyporheic flow paths. Nitrate in surface channel stimulates new primary productivity by algae. After Valett et al. 1994

Abundance of algae in streambed sediments, as indicated by concentration of chlorophyll a, was markedly greater in areas of upwelling (where groundwater moved upward through the sediments) than in areas of downwelling. After Valett et al. 1994

Hyporheic zone: under and adjacent to stream with active exchange of water Permafrost  Active Layer

In dry valley streams the “hyporheic zone” is observed as a wetted zone adjacent to stream

Dry valley streams flow through porous alluvium with high rates of “hyporheic exchange”.

Experimental enrichment with nitrate and phosphate: 1.5 hrs

Sampling Green Creek during experiment

Stream algal mats win! Added nutrients are taken up and do not reach the lake.

hyporheic zone feedbacks to terrestrial landscape: example from southeast Alaska Pacific salmon spend most of their lives growing at sea before returning to fresh water to spawn and die in their natal streams, carrying marine-derived nitrogen in their body tissues. Returning salmon provide a seasonal food source for birds & mammals, and nutrients from decaying salmon carcasses are incorporated into freshwater biota at various trophic levels. From this nutrient subsidy, growth rates are significantly increased in trees near spawning streams. MDN be transferred from spawning streams to riparian forests by flooding which deposits salmon carcasses on stream banks & through the transfer of dissolved nutrients from spawning by hyporheic exchange.

Shading DOC. inputs Sediment, nutrient filtration Bank stabilization Flooding Bank erosion Sediment deposition MDN fertilization of riparian vegetation Materials carried downstream MDN carried upstream Enhanced riparian growth Changes in spp. composition, diversity, soil chemistry Hyporheic Slide courtesy of Bob Naiman, University of Washington

Where does hyporheic exchange take place? example from southwest Alaska Lateral nutrient transfers occur in the hyporheic zone & in adjacent riparian floodplains along spawning reaches. Hyporheic storage & re-release of MDN is an important mechanism by which nutrients are retained over winter within stream ecosystems and subsequently made available to primary producers the following growing season.

hydraulic head gradients in a well network Slide courtesy of Tom O’Keefe & Rick Edwards, University of Washington

DO gradients illustrating hyporheic exchange Slide courtesy of Tom O’Keefe & Rick Edwards, University of Washington

Photo courtesy of Bob Naiman, University of Washington

Loss of water from stream reach to the hyporheic zone commonly related to meanders in streambed Figure from Winter et al. 2000

Where does hyporheic exchange take place? example from St. Kevin Gulch, CO A study of a third order gravel bed stream in the Colorado Rocky Mountains, characterized by pools & riffles, indicates geomorphic controls on where hyporheic exchange takes place.

Inflow of water from the hyporheic zone to the stream was greatest at the downstream end of riffles. After Harvey & Bencala, 1993

From Harvey & Bencala, 1993 Stream water flows into the subsurface beneath and to the side of steep sections of streams (riffles), & ground water enters streams most readily at the upstream end of deep pools

Hyporheic exchange caused to a large extent by the irregular topography of the streambed, which creates pools & riffles characteristic of mountain streams. Channel irregularity is an important control on the location of groundwater inflow to streams and on the size of the hyporheic zone in mountain streams because changes in slope determine the length & depth of hyporheic flow paths. important points about controls on where hyporheic exchange takes place Harvey & Bencala, 1993

Loss of water from stream reach to the hyporheic zone commonly related to abrupt changes in slope of the streambed Figure from Winter et al. 2000

How extensive can a hyporheic zone be? Depending on the type of sediment in the streambed and banks, the variability in slope of the streambed, and the hydraulic gradients in the adjacent ground-water system, the hyporheic zone can be as much as several feet in depth and hundreds of feet in width. The dimensions of the hyporheic zone generally increase with increasing width of the stream and permeability of streambed sediments.

A large, 9 th order stream with permeable, coarse- grained, alluvial deposits in the channel & floodplain HZ width = hundreds of m Nitrogen coming in from regional groundwater was removed in the HZ HZ gains/losses in flow over reaches of 1-2 km were on the order of 5% of streamflow Example of large hyporheic zone: Willamette River, OR

CENTRAL QUESTION What are some common methods used to quantify hyporheic flowpaths?

Subsurface measurements in piezometers Water table Unconfined aquifer Water levels in piezometers are located at the water table. This level is a measure of the hydraulic head, or potential at that point.

upwelling & downwelling can be inferred with piezometer monitoring data Horizontal flow direction Vertical flow direction

Hydrometric measurements at Rio Calavares, NM Photo courtesy of Michelle Baker, Utah State University

Injection & transport of a solute tracer in the stream A tracer is released, and measurements of its passage are made at a location downstream. What will a graph of tracer concentration over time look like at the downstream monitoring point?

Injection & transport of a solute tracer in the stream The concentration will rise, reach a plateau, and then decline as the pulse passes the point of monitoring. The solute disperses from its point of release due to the force of the current (advection) and diffusion and turbulent mixing throughout the stream.

Dye tracer illustrates water storage at channel margins: 1) sides are initially dye-free Photo courtesy of Jud Harvey, USGS, Reston, VA

Dye tracer illustrates water storage at channel margins: 2) all parts of channel have dye Photo courtesy of Jud Harvey, USGS, Reston, VA

Dye tracer illustrates water storage at channel margins: 3) sides retain dye longer than center channel Photo courtesy of Jud Harvey, USGS, Reston, VA

Five storage zones observed in a small mountain stream (Bencala & Walters 1983): turbulent eddies generated by large-scale bottom irregularities large but slowly moving recirculating zones along the sides of pools small but rapidly recirculating zones behind flow obstructions, especially in riffles side pockets flow in and out of beds of coarse substrate. Transient Storage: the temporary retention of solutes in slowly moving or stationary water, and the eventual movement of solutes and water back into the stream channel.

As a first approximation, this curve can be achieved with a basic equation describing advection & dispersion, taking into account stream dimensions & water velocity. Change in solute concentration over time = ∂ C / ∂ t = - U ∂ C / ∂ x + D ∂ 2 C / ∂ x 2 –1st term describes downstream advection and is proportional to water velocity, u. –2 nd term describes mixing of the solute randomly throughout the mass according to a dispersion coefficient D modelling the injection & transport of a solute tracer in the stream

More complicated models are needed to account for additional variables such as groundwater and tributary inputs, channel storage and subsurface flow. Inclusion of terms for transient storage is necessary for the description of solute dynamics in small streams and their hyporheic zones. modelling the injection & transport of a solute tracer in the stream OTIS: One-dimensional Transport with Inflow and Storage— Conceptual Model (OTIS, Runkel 1998) Represents stream transport, inflow, outflow, & exchange

Passage of a the tracer, including transient storage is given by: ∂ C/ ∂ t = -U ∂ C/ ∂ x + D ∂ 2 /C/ ∂ x 2 + α(C s - C) ∂ C s / ∂ t = - α A/A s (C s – C) Where A s is the cross-sectional area of a hypothetical storage zone. The rate of dispersion of solute in or out of this zone is proportional to the difference between solute concentration in the storage zone (C s ) and the water column (C) and an exchange coefficient (α). advectiondispersion exchange modelling the injection & transport of a solute tracer in the stream

Adding a transient storage term permits the model to account for significant features of the of a solute pulse that the earlier equation is unable to mimic. Specifically, measured passage of a tracer pulse usually shows the rising shoulder of the actual pulse to be more gradual and the descending tail to be prolonged relative to the symmetrical curve generated by the earlier equation.

The storage zone component of the model is an abstraction. In contrast to cross-sectional area (A) of the stream channel, which can be measured directly, storage zone area (A s ) is determined by fitting the model to observed solute dynamics. Nonethelesss, storage zones exist and are numerous. Model estimates of As provide a useful index of the size of the transient storage effect.

These models are empirically useful descriptions of observed dynamics, in which transient storage clearly takes place. Example: Rhodamine WT tracer Experiment in Clackamas River, OR (Laenen & Risley 1997). Downstream data are better when simulated with hyporheic exchange considered in the modeling.

A study of a coastal mountain stream in northern California indicated that transport of dissolved oxygen, dissolved carbon, and dissolved nitrogen in stream water into the hyporheic zone stimulated uptake of nitrogen by microbes and algae attached to sediment. A model simulation of nitrogen uptake indicated that both the physical process of water exchange between the stream and the hyporheic zone and the biological uptake of nitrate in the hyporheic zone affected the concentration of dissolved nitrogen in the stream. Hyporheic zone as a sink for nutrients: Example w/ modelling to interpret field data from Little Lost Man Creek, CA Kim et al. 1992

Nitrate injected into stream was stored and taken up by algae and microbes in the hyporheic zone. After Kim et al. 1992

The ratio of storage zone area to stream cross sectional area As/A exhibits a strong positive relation with channel friction factor. (From Harvey & Wagner 2000) Channel friction factor = 8gds/u 2 (where g is gravitational acceleration, d is stream depth, s is streambed slope, and u is stream velocity).

references Bencala K (1993). A perspective on stream-catchment connections. Journal of the North American Benthological Society 12(1): Eagleson PS (2002). Ecohydrology: Darwinian expression of vegetation form and function. Cambridge University Press, 443pp. Harvey JW & KE Bencala (1993). The effect of streambed topography on surface-subsurface water exchange in mountain catchments. Water Resources Research 29(1): Harvey JW & BJ Wagner (2000). Quantifying hydrologic interactions between streams and their subsurface hyporheic zones. pp in: JB Jones & PJ Mulholland, eds., Streams and Ground Waters. Academic Press. Helfield JM & RJ Naiman (2001). Effects of salmon-derived nitrogen on riparian forest growth and implications for stream productivity. Ecology 82(9): Hinkle SR, JH Duff, FJ Triska, A Laenen, EB Gates, KE Bencala, DE Wentz, & SR Silva (2001). Linking hyporheic flow and nitrogen cycling near the Willamette River – a large river in Oregon, USA. Journal of Hydrology 244: Nuttle WK (2002) Eco-hydrology’s past and future in focus. Eos 83(7 May): 205. O’Keefe TC & RT Edwards (2002). Evidence for hyporheic transfer and removal of marine-derived nutrients in a sockeye stream in southwest Alaska. American Fisheries Society Symposium 33: Valett HM, SG Fisher, NB Grimm, & P Camill (1994). Vertical hydrologic exchange and ecologic stability of a desert stream ecosystem. Ecology 75:548–560. Rodriguez-Iturbe I (2000). Ecohydrology: a hydrologic perspective of climate–soil–vegetation dynamics. Water Resources Research 36:3–9. Runkel (1998). One-dimensional transport with inflow and storage (OTIS): A solute transport model for streams and rivers. US Geological Survey, Water Resources Investigations Report Triska FJ, VC Kennedy, RJ Avanzino, GW Zellweger, & KE Bencala (1989). Retention and transportation of nutrients in a third-order stream in northwestern California: hyporheic processes. Ecology 70: Kim BKA, AP Jackman, & FJ Triska (1992). Modeling biotic uptake by periphyton and transient hyporheic storage of nitrate in a natural stream. Water Resources Research 28(10): 2743–2752 Winter TC, JW Harvey, OL Franke, & WM Alley (1998). Ground water and surface water: A single resource. US Geological Survey, Circular 1139, 87 pp.