Showing papers by "Michael N. Gooseff published in 2011"

Rethinking hyporheic flow and transient storage to advance understanding of stream‐catchment connections

Abstract: [1] Although surface water and groundwater are increasingly referred to as one resource, there remain environmental and ecosystem needs to study the 10 m to 1 km reach scale as one hydrologic system. Streams gain and lose water over a range of spatial and temporal scales. Large spatial scales (kilometers) have traditionally been recognized and studied as river-aquifer connections. Over the last 25 years hyporheic exchange flows (1–10 m) have been studied extensively. Often a transient storage model has been used to quantify the physical solute transport setting in which biogeochemical processes occur. At the longer 10 m to 1 km scale of stream reaches it is now clear that streams which gain water overall can coincidentally lose water to the subsurface. At this scale, the amounts of water transferred are not necessarily significant but the exchanges can, however, influence solute transport. The interpretation of seemingly straightforward questions about water, contaminant, and nutrient fluxes into and along a stream can be confounded by flow losses which are too small to be apparent in stream gauging and along flow paths too long to be detected in tracer experiments. We suggest basic hydrologic approaches, e.g., measurement of flow along the channel, surface and subsurface solute sampling, and routine measurements of the water table that, in our opinion, can be used to extend simple exchange concepts from the hyporheic exchange scale to a scale of stream-catchment connection.

Bacterial Community Structure Along Moisture Gradients in the Parafluvial Sediments of Two Ephemeral Desert Streams

Abstract: Microorganisms inhabiting stream sediments mediate biogeochemical processes of importance to both aquatic and terrestrial ecosystems. In deserts, the lateral margins of ephemeral stream channels (parafluvial sediments) are dried and rewetted, creating periodically wet conditions that typically enhance microbial activity. However, the influence of water content on microbial community composition and diversity in desert stream sediments is unclear. We sampled stream margins along gradients of wet to dry sediments, measuring geochemistry and bacterial 16S rRNA gene composition, at streams in both a cold (McMurdo Dry Valleys, Antarctica) and hot (Chihuahuan Desert, New Mexico, USA) desert. Across the gradients, sediment water content spanned a wide range (1.6–37.9% w/w), and conductivity was highly variable (12.3–1,380 μS cm−2). Bacterial diversity (at 97% sequence similarity) was high and variable, but did not differ significantly between the hot and cold desert and was not correlated with sediment water content. Instead, conductivity was most strongly related to diversity. Water content was strongly related to bacterial 16S rRNA gene community composition, though samples were distributed in wet and dry clusters rather than as assemblages shifting along a gradient. Phylogenetic analyses showed that many taxa from wet sediments at the hot and cold desert site were related to, respectively, halotolerant Gammaproteobacteria, and one family within the Sphingobacteriales (Bacteroidetes), while dry sediments at both sites contained a high proportion of taxa related to the Acidobacteria. These results suggest that bacterial diversity and composition in desert stream sediments is more strongly affected by hydrology and conductivity than temperature.

Water tracks and permafrost in Taylor Valley, Antarctica: Extensive and shallow groundwater connectivity in a cold desert ecosystem

Abstract: Water tracks are zones of high soil moisture that route water downslope over the ice table in polar environments. We present physical, hydrological, and geochemical evidence collected in Taylor Valley, McMurdo Dry Valleys, Antarctica, which suggests that previously unexplored water tracks are a significant component of this cold desert land system and constitute the major flow path in a cryptic hydrological system. Geological, geochemical, and hydrological analyses show that the water tracks are generated by a combination of infiltration from melting snowpacks, melting of pore ice at the ice table beneath the water tracks, and melting of buried segregation ice formed during winter freezing. The water tracks are enriched in solutes derived from chemical weathering of sediments as well as from dissolution of soil salts. The water tracks empty into ice-covered lakes, such as Lake Hoare, resulting in the interfingering of shallow groundwater solutions and glacier-derived stream water, adding complexity to the geochemical profile. Approximately four orders of magnitude less water is delivered to Lake Hoare by any given water track than is delivered by surface runoff from stream flow; however, the solute delivery to Lake Hoare by water tracks equals or may exceed the mass of solutes delivered from stream flow, making water tracks significant geochemical pathways. Additionally, solute transport is two orders of magnitude faster in water tracks than in adjacent dry or damp soil, making water tracks “salt superhighways” in the Antarctic cold desert. Accordingly, water tracks represent a new geological pathway that distributes water, energy, and nutrients in Antarctic Dry Valley, cold desert, soil ecosystems, providing hydrological and geochemical connectivity at the hillslope scale.

How can subsurface modifications to hydraulic conductivity be designed as stream restoration structures? Analysis of Vaux's conceptual models to enhance hyporheic exchange

Abstract: [1] Despite the growing interest in hyporheic exchange and the associated stream ecosystem processes, few studies consider restoration of hyporheic exchange as a design goal. Here we study the design of three types of subsurface structures for hyporheic restoration after conceptual designs published over 40 years ago. Vaux's designs involve modifying the subsurface with low or high hydraulic conductivity material placed at the streambed or adjacent to a confining layer below the stream. In this preliminary analysis of subsurface structure design we use two-dimensional groundwater flow modeling of structures to simulate structure performance in plane bed streams for ranges of structure geometric design and hydraulic conductivities. Structure performance is evaluated on the basis of total streambed flux, physical extent of hyporheic flow paths created, and residence time distributions along flow paths modified by the structures. High hydraulic conductivity structures bend flow paths toward and through the structures themselves; performance is controlled by the structure hydraulic conductivity. Results show low hydraulic conductivity structure performance is insensitive to the structure material; hyporheic exchange is created by deflecting flow paths away from the structure itself. Time scales of simulated exchange are great enough to promote nitrification, denitrification, respiration, and thermal buffering in the subsurface, though these processes will also be controlled by site-specific chemical and biological factors. General design recommendations for specific restoration objectives are presented. Results of this study can be extrapolated to further understand the interaction of natural subsurface heterogeneities (e.g., clay and gravel deposits and bedrock knickpoints) and flow fields in creating hyporheic flow paths.

Separation of river network–scale nitrogen removal among the main channel and two transient storage compartments

Abstract: [1] Transient storage (TS) zones are important areas of dissolved inorganic nitrogen (DIN) processing in rivers. We assessed sensitivities regarding the relative impact that the main channel (MC), surface TS (STS), and hyporheic TS (HTS) have on network denitrification using a model applied to the Ipswich River in Massachusetts, United States. STS and HTS connectivity and size were parameterized using the results of in situ solute tracer studies in first- through fifth-order reaches. DIN removal was simulated in all compartments for every river grid cell using reactivity derived from Lotic Intersite Nitrogen Experiment (LINX2) studies, hydraulic characteristics, and simulated discharge. Model results suggest that although MC-to-STS connectivity is greater than MC-to-HTS connectivity at the reach scale, at basin scales, there is a high probability of water entering the HTS at some point along its flow path through the river network. Assuming our best empirical estimates of hydraulic parameters and reactivity, the MC, HTS, and STS removed approximately 38%, 21%, and 14% of total DIN inputs during a typical base flow period, respectively. There is considerable uncertainty in many of the parameters, particularly the estimates of reaction rates in the different compartments. Using sensitivity analyses, we found that the size of TS is more important for DIN removal processes than its connectivity with the MC when reactivity is low to moderate, whereas TS connectivity is more important when reaction rates are rapid. Our work suggests a network perspective is needed to understand how connectivity, residence times, and reactivity interact to influence DIN processing in hierarchical river systems.

Quantifying hyporheic exchange at high spatial resolution using natural temperature variations along a first?order stream

Abstract: Hyporheic exchange is an important process that underpins stream ecosystem function, and there have been numerous ways to characterize and quantify exchange flow rates and hyporheic zone size. The most common approach, using conservative stream tracer experiments and 1?D solute transport modeling, results in oversimplified representations of the system. Here we present a new approach to quantify hyporheic exchange and the size of the hyporheic zone (HZ) using high?resolution temperature measurements and a coupled 1?D transient storage and energy balance model to simulate in?stream water temperatures. Distributed temperature sensing was used to observe in?stream water temperatures with a spatial and temporal resolution of 2 and 3 min, respectively. The hyporheic exchange coefficient (which describes the rate of exchange) and the volume of the HZ were determined to range between 0 and 2.7 × 10?3 s?1 and 0 and 0.032 m3 m?1, respectively, at a spatial resolution of 1–10 m, by simulating a time series of in?stream water temperatures along a 565 m long stretch of a small first?order stream in central Luxembourg. As opposed to conventional stream tracer tests, two advantages of this approach are that exchange parameters can be determined for any stream segment over which data have been collected and that the depth of the HZ can be estimated as well. Although the presented method was tested on a small stream, it has potential for any stream where rapid (in regard to time) temperature change of a few degrees can be obtained.

Hydrological Connectivity of the Landscape of the McMurdo Dry Valleys, Antarctica

Abstract: The McMurdo Dry Valleys (MDV) of Antarctica are composed of nearly 2000 km 2 of ice-free terrain, supporting a vibrant cold desert ecosystem despite harsh conditions. The ecosystem is largely regulated by the hydrologic cycle within the MDV, which is controlled by climate dynamics. The strength and timing of connections among the hydrologic reservoirs of the MDV (atmosphere, glaciers, soils and permafrost, streams and their hyporheic zones, and lakes) are dependent upon daily, seasonal, and annual surface energy balance. For example, glacier melt occurs during short periods in the summer, providing stream flow to closed-basin lakes. Similarly, the magnitude of sublimation of snow on valley floors and perennial ice covers on lakes is a function of wind, temperature, radiation, and atmospheric water content (humidity). Here, we describe these reservoirs and connections across the landscape to provide an overview of our current understanding of the system, as it is poised to change in response to changing climate in the coming decades. Measurement of hydrologic fluxes and states of hydrologic reservoirs in the MDV provides both a context for quantifying responses to climate change and a careful characterization of the potential direct drivers of ecosystem response. The MDV also provide a unique real-world laboratory in which to study fundamental hydrologic processes (with the exception of rainfall).

Residence time distributions in surface transient storage zones in streams: Estimation via signal deconvolution

Abstract: [1] Little is known about the impact of surface transient storage (STS) zones on reach-scale transport and the fate of dissolved nutrients in streams. Exchange with these locations may influence the rates of nutrient cycling often observed in whole-stream tracer experiments, particularly because they are sites of organic matter collection and lower flow velocities than those observed in the thalweg. We performed a conservative stream tracer experiment (slug of dissolved NaCl) in the Ipswich River in northeastern Massachusetts and collected solute tracer data both in the thalweg and adjacent STS zones at three locations in a fifth-order reach. Tracer time series observed in STS zones are an aggregate of residence time distributions (RTDs) of the upstream transport to that point (RTDTHAL) and that of the temporary storage within these zones (RTDSTS). Here we demonstrate the separation of these two RTDs to determine the RTDSTS specifically. Total residence times for these individual STS zones range from 4.5 to 7.5 h, suggesting that these zones have the potential to host important biogeochemical transformations in stream systems. All of the RTDSTS show substantial deviations from the ideal prescribed by the two-state (mobile/immobile) mass transfer equations. The deviations indicate a model mismatch and that parameter estimation based on the mass transfer equations will yield misleading values.

Implications of meltwater pulse events for soil biology and biogeochemical cycling in a polar desert

Abstract: The McMurdo Dry Valleys are one of the most arid environments on Earth. Over the soil landscape for the majority of the year, biological and ecosystem processes in the dry valleys are constrained by the low temperatures and limited availability of water. The prevalence of these physical limitations in controlling biological and ecosystem processes makes the dry valleys a climatesensitive system, poised to experience substantial changes following projected future warming. Short-duration increases in summer temperatures are associated with pulses of water from melting ice reserves, including glaciers, snow and permafrost. Such pulses alter soil geochemistry by mobilizing and redistributing soil salts (via enhanced weathering, solubility and mobility), which can alter habitat suitability for soil organisms. Resulting changes in soil community composition or distribution may alter the biogeochemical processes in which they take part. Here, we review the potential impacts of meltwater pulses and present new field data documenting instances of meltwater pulse events that result from different water sources and hydrological patterns, and discuss their potential influence on soil biology and biogeochemistry. We use these examples to discuss the potential impacts of future climate change on the McMurdo Dry Valley soil ecosystem. Keywords: Water pulse; climate change; polar desert; International Polar Year; discrete warming events; soil biogeochemistry (Published: 19 December 2011) Citation: Polar Research 2011, 30 , 14555, DOI: 10.3402/polar.v30i0.14555

Hydrology and Biogeochemistry Linkages

Abstract: This chapter provides an overview of the linkages between hydrology and biogeochemistry in terrestrial and aquatic systems. Selected topics include hydrological pathways on drainage basin slopes, mountain environments, within-river (or in-stream) processes, wetlands, groundwater (and groundwater–surface water interactions), and lakes. Beginning from catchment headwaters, This chapter introduces mechanisms delivering water from hillslopes to stream channels, highlighting the relative importance of biogeochemical processes along hydrological pathways. It considers processes affecting components of the water budget, including snow formation and ablation processes, and interactions with the soil below snow cover and during snowmelt. It presents the concept of nutrient spiraling and the importance of temperature and stream flow variability on biogeochemistry, as well as groundwater–surface water interactions through hyporheic and riparian zones. This chapter contrasts important processes in hydrologically isolated wetlands with those temporally connected to streams and rivers. It addresses stream and groundwater inputs, stratification, and within-lake processes, interactions with sediments, and a discussion about limiting nutrients. This chapter presents information about typical reactions controlled by hydrological pathways, lithology (mineralogy) and biota, the importance of residence time in biogeochemical evolution, and linkages between groundwater and surface water. An example is given of the effects of human activities on these linkages, focusing on acidic atmospheric deposition.