The focus of our study was to investigate inorganic and organic carbon dynamics in the hyporheic zone of a headwater catchment in the Western Cascade Mountains of Oregon. We collected water samples from a well network that spanned the full width of the riparian corridor at the base of the catchment during baseflow periods in each of 7 months. In our analysis, we sought to evaluate the influence of seasonal, physical, and biogeochemical processes on carbon cycling within this hyporheic environment.
This study investigated carbon dynamics in the hyporheic zone of a steep, forested catchment in the Cascade Mountains of western Oregon, USA. Water samples were collected monthly from a headwater stream and well network during baseflow conditions from July to December 2013 and again in March 2014. We also sampled during one fall storm event, collecting pre-storm, rising leg, and extended high flow samples. The well network is located at the base of Watershed 1 (WS1) of the H.J. Andrews Experimental Forest and spans the full width of the floodplain (~14 m) along a 29 m reach of stream. We measured pH, temperature, water level, major anions, major cations, DOC, DIC, and total alkalinity.
We conducted all fieldwork in Watershed 1 at the H.J. Andrews Experimental Forest. In order to prepare the well network for sampling, we cleared all wells of accumulated sediment by flushing them once with pressurized stream water. We cleaned one half of the network in early June 2013 in preparation for a partial sampling run, and washed the remaining wells in mid-July 2013. In each case, the cleaning was performed at least a week in advance of sample collection so that water chemistry would recover and thus reflect in-situ hyporheic conditions.
On the afternoon prior to each sampling run, we recorded the pH and temperature of each well and the stream using an YSI 60 probe. We also determined the depth to water in each well using a tape measure and wet-erase marker. Following this, we removed 700 mL from each of the wells using the designated purge syringes and tubing.
The following morning, we sampled each of the 28 wells, the stream, and the hillslope well, collecting one field duplicate for every ten samples. We extracted water from the wells using the acid washed 60 mL syringes, stopcocks, and 0.635 cm sample tubing. Prior to sample collection, we rinsed the sample tubing and syringes three times with approximately 180 mL of well water. We used an additional 60 mL of water to rinse the prepared GFF filters and the acid washed HDPE bottles. We then collected 250 mL of water for DOC, anion, cation, and alkalinity analysis, which we filtered in the field through the prepared glass microfiber filters into the HDPE bottles. We sampled stream water using this same technique. On a few occasions, 250 mL of unfiltered stream and well water were collected for TOC analysis. We collected sample aliquots for DIC analysis last so as to minimize exposure of the water to the atmosphere, and preserved each sample in an individual, locked, airtight syringe. We transported all samples back to the lab on ice and kept them in cold storage until analysis.
We sampled the WS1 well network at monthly intervals from July to December 2013 and then again in March 2014, focusing our sampling during baseflow or near-baseflow periods after relatively dry antecedent conditions. We obtained discharge, stream temperature, and stream conductivity data from the WS1 gaging station, located approximately 50 m below the well network. We used precipitation and temperature data recorded at the PRIMET meteorological station at the H.J. Andrews headquarters, located approximately 0.6 km from the WS1 gaging station.
FOR THE STORM:
Our sampling was conducted from November 15th – 18th. On the afternoon of November 14th, 2013, we recorded the pH and temperature of each well and the stream using an YSI 60 probe. We then removed 700 mL from each of the wells using the designated purge syringes and tubing. On the morning of the 15th, when the storm had not yet begun and the system was still at a baseflow discharge, we sampled wells D5, D6, D7, E4, G1, G2, G3, G5, G6 and the hillslope well, collecting one field duplicate. We sampled the stream at both the beginning and end of the sampling run. We collected all samples according to the method outlined in Corson-Rikert (2014).
After the initial pre-storm sampling run on the morning of the 15th, we used real-time discharge data from the WS1 gaging station (located approximately 50 m below the well network) and precipitation data from the PRIMET meteorological station (located approximately 0.6 km from the gaging station) in order to mark the onset of the storm and track the progression of the stream response. We conducted our second sampling run at the tail end of the rising leg early in the morning of November 16th. We sampled for a third time during an initial peak in the WS1 hydrograph on the afternoon of the 16th. We performed the final sampling run after a sustained period of high flow, during the afternoon of November 17th. We used the same procedure and sampled the same wells during all four sampling runs.
Prior to fieldwork, we acid washed all equipment for field sample collection and lab analysis in a 10% v/v HCl acid bath according to the CCAL quality assurance plan (CCAL, unpublished, 2013). This equipment included 40 mL borosilicate vials, 250 mL Nalgene HDPE bottles, VWR 60 mL syringes with BD Luer-Lok tips, Cole-Parmer polycarbonate stopcocks with Luer connections, a 4.7 cm filter apparatus, and 0.635 cm (1/4 inch) sample tubing. In addition, we baked the 40 mL borosilicate vials that were allocated for DOC analysis at 550 ºC for three hours (CCAL, unpublished, 2013). We rinsed the 140 mL syringes and 0.635 cm tubing used for purging the wells in DI water and dried them in a low-temperature oven between sampling runs. We prepared Whatman 4.7 cm grade GF/F glass microfiber filters by rinsing them with 1 L of DI water, drying them, and baking them in a muffle furnace at 500 ºC for 3 hours (CCAL, unpublished, 2013). We stored prepared filters in clean, individual foil packets. Note that the use of trade or firm names in this publication is for reader information and does not imply endorsement by the US Department of Agriculture of any product or service.
We performed all analytical work in the IWW Collaboratory at Oregon State University. Methods for these analyses are presented below. CCAL standard operating procedures are developed primarily from the cited APHA methods, but comparable EPA methods are noted for reference. The format of all methods citations are as follows: (CCAL standard operating procedure, APHA method, EPA method, method detection limit).
Immediately prior to analysis, we filtered the DIC sample aliquots through 25 mm diameter VWR 0.45 µm nylon syringe filters with polypropylene housing into acid washed 40 mL borosilicate vials. We filled the vials at an angle and capped them as soon as an inverted meniscus was formed, so as to limit atmospheric exposure. We analyzed the filtered samples on a Shimadzu TOC-VSCH Combustion Carbon Analyzer within 72 hours (CCAL 21A.0, n/a, n/a, 0.05 mg/L). We modified the procedure slightly in order to account for higher concentrations of DIC in hyporheic water by using ten standards ranging in concentration from 0 to 20 ppm and including both 1 and 10 ppm check standards.
We used aliquots of the field-filtered 250 mL sample for DOC, anion, cation, and alkalinity analysis. We used the unfiltered stream and well water, when it was collected, to analyze TOC content. We determined concentrations of DOC and TOC using a Shimadzu TOC-VSCH Combustion Carbon Analyzer (CCAL 20A.2, APHA 5310B, EPA 415.1, 0.05 mg C L-1). We measured major cations K+, Na+, Mg+2, and Ca+2 on a Perkin-Elmer Atomic Absorption Spectrometer, a Perkin-Elmer AAnalyst-100 (CCAL 60B.1, APHA 3111, EPA 7000B, K+: 0.03 mg L-1, Na+: 0.01 mg L-1, Mg+2: 0.02 mg L-1, Ca+2: 0.06 mg L-1). We determined concentrations of major anions NO3-, SO4-2, Cl-, and PO4-3 on a Dionex 1500 Ion Chromatograph (CCAL 50B.1, APHA 4110B, EPA 9056A, 0.01 mg L-1). We measured total alkalinity by titrating all samples to a pH of 4.5 on a Radiometer TIM840 AutoTitrator (CCAL 10C.0, APHA method 2320 - modifications: use 0.02N Na2CO3 and 0.02N H¬2SO4, no EPA method, 0.2 mg CaCO3 L-1).
We conducted all preparatory and analytical lab work at the Institute for Water and Watersheds Collaboratory at Oregon State University using operating procedures developed by the Oregon State University and United States Forest Service Cooperative Chemical Analytical Laboratory (CCAL) and followed the CCAL quality assurance plan.
This study was conducted in the lower portion of Watershed 1 (WS1), a study watershed in the H.J. Andrews Experimental Forest, located in the Western Cascades of Oregon, USA (44º 12’ 28.0” N, 122º 15’ 30.0” W). Watershed 1 is a steep, forested catchment that is 95.9 ha in size, and ranges in elevation from 450 to 1027 m. The climate is characterized by cool, wet winters and warm, dry summers. At lower elevations, air temperatures range from an average of 1 ºC in January to 18ºC in July. The 230 cm of annual precipitation falls primarily as rain from November to March. Snow occasionally accumulates, but WS1 lies within the transient snow zone, where snow accumulates during cold winter storms, but melts during warm periods or warmer storms, so that snow packs do not persist for the entire winter. As a result, peak streamflow in WS1 occurs anytime throughout the winter and declines in summer months. Streamflow becomes so low in July, August, and September that surface flow cannot be continuously sustained, and the stream becomes spatially intermittent.
Throughout WS1, the soils are gravel clay loam, and tend to be shallow (0.5 – 2 m) and porous, allowing high rates of infiltration (Rothacher et al., 1967; Dyrness, 1969). Tuffs and breccias of the Oligocene to lower Miocene Little Butte Formation underlie the middle and lower portions of the watershed, while the upper northeast corner is underlain by an andesitic flow (Swanson and James, 1975).
From 1962 to 1966, WS1 was 100% clear-cut using skyline yarding to lift logs clear of the ground and thereby minimize soil disturbance as logs were removed from the watershed. In 1966, the logging debris (slash) was burned (Levno and Rothacher, 1969; Halpern and Franklin, 1990). Today, dense stands of Douglas fir and hemlock dominate on hillslopes. Red alder dominates the riparian zone, although maple, cottonwood, and dogwood are also present (Rothacher et al., 1967; Halpern and Franklin, 1990; Johnson and Jones, 2000). The alders established after the logging operation, and today are being over-topped by Douglas fir. As a result, many have died or fallen in recent years.
The stream channel in WS1 is steep and usually confined. It has been shaped by debris flows, which have scoured the stream to bedrock in places. Reaches where colluvium is deposited are less constrained (Wondzell, 2006). Our study site is located in a zone of colluvial deposition near the base of WS1. The channel remains steep, with an average longitudinal gradient of 14%. The poorly sorted colluvial sediment is up to 2 m thick across the 14 m wide valley floor (Wondzell, 2006). The channel is broken into a series of pools and steps that have formed over logs and boulders (Wondzell, 2006). These steps drive 50% of hyporheic exchange between the stream and subsurface (Kasahara and Wondzell, 2003). During high and low baseflow, down-valley hydraulic gradients along this reach average 1.4 times steeper than cross-valley gradients (Wondzell, 2006). This leads to the development of extended flow paths that are parallel to the stream and to the predominance of long-timescale hyporheic exchange (Wondzell, 2006), with median residence times estimated at 17 hours (Kasahara and Wondzell, 2003). Despite these long residence times, hyporheic water in WS1 is generally oxic (Serchan, unpublished data, 2014)
The well network is located in the 14 m wide riparian zone at the base of WS1, and spans 29 m of stream length (Wondzell, 2006). When it was installed in 1997, it contained 30 shallow (~1 – 1.7 m deep) riparian wells and 7 in-stream piezometers (2006). This study was able to sample 24 wells and 4 piezometers – 9 of the original were cracked, missing, or had gone dry. The wells and piezometers are arrayed in six transects that span the width of the 14 m wide valley floor, perpendicular to the direction of flow. Both wells and piezometers are constructed of 3.175 cm (1 1/4 inch) schedule 40 PVC. An array of drilled holes serves as a screen along the bottom 50 cm of the wells and bottom 5 cm of the in-stream piezometers (Wondzell, 2006). The deepest well extends to 1.7 m, but the majority are approximately 1 m deep (Wondzell, 2006). We also sampled one hillslope well, located approximately 150 m up the watershed at the base of a hillslope hollow, which we installed during the summer of 2013.
