Changes In Stream Temperatures In Response To Restoration Of Groundwater Discharge And Solar Heating In A Culverted, Urban Stream

Boone Creek is a mountainous headwater stream that lies within an urbanized environment in northwestern North Carolina. The primary source of thermal pollution in Boone Creek is the urban infrastructure, which affects stream temperatures through (1) heated runoff, which creates temperature surges and (2) the elimination of groundwater–surface water interactions. In this study, we use a Monte Carlo thermal mixing model to predict the thermal impact of removing a 700-m-long culvert. Our thermal mixing model balances stream discharge and temperatures with surface-heat exchange parameters and restored base?ow. We calculate the daily-average groundwater discharge velocity at 15 locations in the stream using signal decay in streambed temperatures, and utilize a Monte Carlo implementation of the heterogeneous base?ow ?eld in the thermal mixing model. We also calculate surface-heat exchange per unit area for conditions upstream and downstream of the existing culvert and utilize those values in the thermal mixing model. Our modeled temperatures suggest a decrease in summer stream temperatures downstream of the culvert that average 1.35 °C and 1.17 °C for upstream and downstream surface-heat exchange conditions, respectively. The results of our study have implications for the balance between base?ow and the urban infrastructure in any high-gradient urban stream that experiences similar thermal effects

Riparian Biogeochemical Hot Moments Induced By Stream Fluctuations

Hyporheic exchanges in riparian zones induced by stream stage fluctuations, referred to as bank storage, can influence contaminant transport and transformation when mixing of groundwater and surface waters with distinct chemical signatures occur, which might lead to a high biochemical activity. The effect of bank storage on nutrient transport was analyzed here using a two-dimensional, variably saturated and multi-species reactive transport model, which accounted for the water flow and solute transport and reactions within riparian zones. After verification with field observations, our model demonstrated that high biogeochemical activities occurred at the near-stream riparian zone during stage fluctuation, a process referred to as bank storage hot moment (BSHM). We used Monte Carlo simulations to study the uncertainty of BSHM and related nutrient dynamics to biogeochemical and hydrological factors. The results indicated that stream fluctuations can lead to maximum bank storage volume ranging from 0 to 259 m3 m1 of stream linear length (median ¼ 9.7 m3 and SD ¼ 53.2 m3). Taking denitrification as an example, BSHM can lead to considerable NO3 removal with a median removal rate of 2.1 g d1 and SD of 17.2 g d1 per meter of stream linear length. The NO3 uptake velocity (median ¼ 2.7 105 and SD ¼ 2.4 104 mmin1) was comparable to that of in-stream transient storage from the literature. This result suggests that BSHM may be a significant process contributing to the nutrient budget at the ecosystem level. Finally, a theoretical framework representing the coupled hydrobiogeochemical controls on riparian hot spots was developed to help predicting when BSHM can become important in a particular stream

The Role of Overwash in the Evolution of Mixing Zone Morphology Within Barrier Islands

Overwash is a major controlling factor in the morphology of the mixing zone of coastal aquifers. Conceptual models of the mixing zone describe an interface controlled by tidal oscillations, wave run-up, and other factors; however, few describe the influence of large storm events. In August 1993, Hatteras Island, North Carolina, USA, experienced a 3-m storm surge due to Hurricane Emily. Sound-side flooding infiltrated a wellfield, causing a dramatic increase in TDS levels that persisted for more than 3 years. Two-dimensional simulations with SUTRA, the USGS finite-element model, are calibrated to the TDS breakthrough data of this storm to infer model dispersivity values. Simulations using the calibrated dispersivity values, predicted flooding levels, and 54 years of hurricane records to determine the influence of the overwash events suggest that it is rare for the mixing zone to approximate the conceptual morphology. Even during quiescent periods such as between 1965 and 1975, TDS levels do not return to theoretical levels before being elevated by a subsequent storm event. Thus, while tidal oscillations and other factors are important to mixing zone development, basic wind events and more severe storm events may have more influence and lasting effect on the morphology of the mixing zone

Air-Stream Temperature Correlation In Forested And Urban Headwater Streams In The Southern Appalachians

Air temperature can be an effective predictor of stream temperature. However, little work has been done in studying urban impacts on air-stream relationships in groundwater-fed headwater streams in mountainous watersheds. We applied wavelet coherence analysis to two 13-month continuous (1 hr interval) stream and air temperature datasets collected at Boone Creek, an urban stream, and Winkler Creek, a forest stream, in northwestern North Carolina. The main advantage of a wavelet coherence analysis approach is the ability to analyse non-stationary dynamics for the temporal covariance between air and stream temperature over time and at multiple temporal scales (e.g. hours, days, weeks and months). The coherence is both time and scale-dependent. Our research indicated that air temperature generally co-varied with stream temperature at time scales greater than 0.5 day. The correlation was poor during the winter at the scales of 1 to 64 days and summer at the scales of 1.5 to 4 days, respectively. The empirical models that relate air temperature to stream temperature failed at these scales and during these periods. Finally, urbanization altered the air-stream temperature correlation at intermediate time scales ranging from 2 to 12 days. The correlation at the urban creek increased at the 12-day scale, whereas it decreased at scales of 2 to 7 days as compared with the forested stream, which is probably due to heated surface runoff during summer thunderstorms or leaking stormwater or wastewater collection systems. Our results provide insights into air-stream temperature relationships over both time and scale domains. Identifying controls over time and scales are needed to predict water temperature to understand the future impacts that interacting climate and land use changes will have on aquatic ecosystem in river networks. Copyright © 2014 John Wiley & Sons, Ltd

Effect Of Interannual Climate Oscillations On Rates Of Submarine Groundwater Discharge (Article #2)

Submarine groundwater discharge (SGD) is an important component of the coastal hydrologic cycle, affecting mixing and bio-geochemistry in the nearshore environment. El Niño–Southern Oscillation (ENSO) influences rates of precipitation and groundwater recharge in many regions, including barrier islands of the southeastern U.S. coast; however, the influence of ENSO on SGD is poorly understood for this region. Here we investigate the role of ENSO in controlling recharge and SGD at inter-annual time scales, using modeling results for both real and generic barrier island environments. Results of our 57 year simulations show that the freshwater component of seasonally averaged SGD as well as groundwater discharge velocity, water table elevation, and submarine groundwater recharge are significantly correlated with ENSO for a real barrier island (Hatteras Island, North Carolina) and, under certain conditions, for generics. These correlations persist for lag times as great as 5 months during winter, creating anomalies of up to 35% between El Niño and La Niña conditions and suggesting that both hydrologic cycling and biogeochemical cycling in these systems are significantly influenced by ENSO

Effect Of Interannual And Interdecadal Climate Oscillations On Groundwater In North Carolina (Article #1)

Multi-year climate oscillations such as the El Nino–Southern Oscillation (ENSO) and Pacific Decadal Oscillation (PDO) affect precipitation and stream discharge rates in the western hemisphere. While inferences may be drawn between these hydroclimatological relationships and groundwater conditions, few studies explicitly link groundwater conditions to these cycles. Here we investigate relationships between winter ENSO, PDO, and lagging baseflow rates in the southeastern United States. We find strong correlation between winter ENSO and lagged baseflow in coastal North Carolina which, coupled with anomalies in mean baseflow, decrease with distance inland from the coast. Our results demonstrate that interannual and interdecadal climate oscillations in the Pacific Ocean have a strong effect on hydrological processes in eastern North America despite filtering by the groundwater flow process. These results have implications for water resource availability in regions where water management is complicated by population growth and climatic uncertainty

Bank Thermal Storage As A Sink Of Temperature Surges In Urbanized Streams

A poorly-studied benefit of bank storage is the ability of the streambed to act as a thermal sink to streams influenced by urban runoff (e.g. bank thermal storage. Headwater streams, with their low thermal inertia, are particularly susceptible to thermal pollution. We utilize numerical modeling to quantify the amount of heat exchanged with the subsurface during temperature surges, which we define as greater than a 1 C stream temperature increase in 15 min. We base our study on Boone Creek, a low-order stream in northwestern North Carolina with stream discharge and temperature data dating to March 2006. The catchment is heavily urbanized, and although the stream is of moderate gradient, it is fed by tributaries that lose up to 200 m/km. The combined effect of urbanization and steep gradient produces a flashy response: stream discharge averages 0.10 m3/s, but may increase up to two orders of magnitude during storm events. These events also affect stream and streambed temperatures. Four summers of monitoring (2006–2008, 2010) indicate that 71 temperature surges occurred with a mean temperature increase of 2.39 C and a maximum increase of 6.36 C. We model generic storm events based on typical Boone Creek storms and streambed hydrogeology with the U.S.G.S. finite-difference groundwater flow and heat transport code VS2DH. The one-dimensional model domain includes a diurnally-oscillating stream temperature and specified head at the upper boundary, a constant streambed temperature and head at the lower boundary, and gaining stream conditions. Reference storm simulations use a temperature increase of 3.66 C and a stream stage increase of 0.66 m. Simulations show that at a depth of 4.5 cm, nearly half of the temperature-surge signal has dissipated and lag times are 30 min. By a depth of 9.5 cm, however, peak temperatures are only one-third of storm levels and lag times are 2 h. At depths beyond 49.5 cm, the perturbation is less than 0.1 C and lags the storm event by more than 17.5 h. Storm influence extends to a depth of 2 m and persists for days. Sensitivity simulations suggest that hydraulic conductivity, sediment heat capacity, and thermal conductivity are the most sensitive model parameters. Calculations show that temperature-surge induced heat storage in the simulated streambed is 72% of the heat storage in the stream

On The Interpretation Of Recharge Estimates From Steady-State Model Calibrations

Ground water recharge is often estimated through the calibration of ground water flow models. We examine the nature of calibration errors by considering some simple mathematical and numerical calculations. From these calculations, we conclude that calibrating a steady-state ground water flow model to water level extremes yields estimates of recharge that have the same value as the time-varying recharge at the time the water levels are measured. These recharge values, however, are a subdued version of the actual transient recharge signal. In addition, calibrating a steady-state ground water flow model to data collected during periods of rising water levels will produce recharge values that underestimate the actual transient recharge. Similarly, calibrating during periods of falling water levels will overestimate the actual transient recharge. We also demonstrate that average water levels can be used to estimate the actual average recharge rate provided that water level data have been collected for a sufficient amount of time

Reconstructing Holocene Sea-Level Change From Coastal Freshwater Peat: A Combined Empirical And Model-Based Approach

This paper presents a novel method to reconstruct sea-level change in coastal freshwater back-barrier marshes. Freshwater environments have long been considered to be unsuitable for the reconstruction of Holocene sea level changes as they provide limiting, rather than precise, sea-level index points. We recorded the stratigraphy of a small beach and back-barrier coastal Phragmites (reed) marsh at Hallsands, south Devon, southwest England, using hand-drilled cores and ground-penetrating radar, and collected five new sea-level index points from the base of a Holocene peat sequence to refine the regional Holocene relative sea-level curve. We demonstrate that the samples, despite their freshwater origin, yield accurate sea-level index points as determined from the quantifiable relationship between tide levels and groundwater. By means of water-table monitoring and groundwater modelling we show that the primary controls on the water table in the marsh are: (1) stratigraphy; (2)peat permeability; and (3) recharge rates in the back-barrier marsh. The five index points document relative sea-level positions between 7200 and 2400 cal yr BP. Three points are in good agreement with previously collected regional data from inter-tidal deposits and two points usefully fill gaps in the existing reconstruction. An amended Holocene relative sea-level curve for south Devon, based on 30 data points, is presented. We conclude that the combined approach of data collection and modelling used in this paper can be applied to similar coastal settings around the world and allows the collection of sea-level index points from locations not previously thought suitable for this purpose