The Hyporheic Zone: Example of a field study Matt Miller Contributions from D. McKnight and N. Mladenov

“Hypo” – Greek, meaning under “Rheo” – meaning current or flow What is the Hyporheic Zone?

The Hyporheic Zone *An area where stream water mixes with groundwater and eventually returns to the stream.

Hyporheic exchange In the bed and banks of streams, water and solutes can exchange in both directions across the streambed. In the bed and banks of streams, water and solutes can exchange in both directions across the streambed. Hyporheic exchange mixes water & solutes from surface and subsurface environments. Hyporheic exchange mixes water & solutes from surface and subsurface environments. Figure from Winter et al. 2000

Some important concepts Hyporheic flowpaths leave and return to the stream many times within a reach Hyporheic flowpaths leave and return to the stream many times within a reach Exchange of surface water back and forth b/w active channel and subsurface is rapid. Exchange of surface water back and forth b/w active channel and subsurface is rapid. Within several km, stream water often completely exchanged with the porewater of the hyporheic zone Within several km, stream water often completely exchanged with the porewater of the hyporheic zone This repeatedly brings stream water into close contact with geochemically- and microbially-active sediment. This repeatedly brings stream water into close contact with geochemically- and microbially-active sediment. Figure from Harvey & Wagner 2000

Mixing of chemically-different waters and chemical transformations Chemical gradients at the interface

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

What are some common methods used to quantify hyporheic flowpaths?

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

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

Modeling the transport of a conservative solute

Main Channel: Storage Zone: Advection Dispersion Lateral inflow Transient storage Decay Modeling Hyporheic Exchange: OTIS

Example of a field study: Green Lakes Valley

Site Description ALBION TOWNSITE LAKE ALBION GL4 GL5 NAVAJO STREAM NIWOT RIDGE

Atmospheric N Deposition

Sources: Groundwater

Sources: Snowmelt

Where does the water end up?

Question 1: How is hyporheic exchange in the upper reaches of the Green Lakes Valley impacting transport of Carbon and Nitrogen to downstream surface waters? Research Question Hypothesis: Given the presence of a porous wetland in the upper reaches of the catchment there will be substantial hyporheic exchange and subsequently high rates of processing of both C and N.

*Tracer Injection w/ LiBr *Modeled with OTIS *Sampled for: -DOC -N species -Li and Br Tracer Injection Experiment

Solute Transport Modeling Results

Reactive Transport Results Water samples are collected and analyzed at downstream sites What will a graph of measured concentrations compared with concentrations predicted with and without in- stream and storage zone reactions look like?

Stream Decay (λ) Storage Zone Decay (λ s ) NO x x NH x x DOC 1.6 x x Redox Index 6.5 x x λCλC λsCλsC Reactive Transport Results

Measured EEM at S3 PARAFAC modeled EEM at S3 Conservative Transport EEM at S3 Reactive Transport Results