Springs Emerging along the Elevation Gradient Indicate Intensive Groundwater-Surface Water Exchange in an Alpine Headwater Catchment, Northwestern China

Groundwater plays an important role in sustaining the streamflow in cold alpine area, but is poorly understood due to a lack of direct access. About 98 groups of springs are observed at the upper reaches of Heihe River Basin, which provide an opportunity to explore the main aquifers. Springs are clustered in three groups according to locations:(1) springs on the moraine and talus deposits;(2) springs at the end of alluvial plain in lower topography;(3) springs along the river bank. The hydrometric, geochemical and isotopic data of springs in a representative catchment were integrated and used to elucidate the groundwater flow paths. Results indicate the Quaternary porous aquifers in the alpine catchment have a profound influence on the regional groundwater flow paths and the groundwater and surface water(GW-SW) interactions. The aquifer consisting of alluvial-pluvial deposits has a great capacity of groundwater storage and plays a vital role in regulating discharge by attenuating the seasonal variation and maintaining the main stream in cold seasons. This is different from the fast recharge and discharge mode of the moraine and talus deposits. Our work highlights the importance of loose deposits in controlling the GW-SW interactions in the cold alpine area.

The influence of slope collapse on water exchange between a pit lake and a heterogeneous aquifer

Due to the increase in open pit mining,pit lakes have become common surface water features,posing a potential risk to subsurface aquifer.In this study,a pit lake–groundwater interaction model is built based on the general program MODFLOW with the LAK3 package.For the first time,the effects of lake-slope collapse and aquifer heterogeneity on pit lake–groundwater interactions are analyzed by dividing the lake into six water exchange zones based on the aquifer lithology and groundwater level.Our investigation and simulations reveal a total water exchange from groundwater to the lake of 349000 m3/a without collapse of the pit lake slope,while the total net water exchange under slope collapse conditions is 248000 m3/a(i.e.,a reduction of 1.40-fold).The monthly net water exchange per unit width from groundwater to the lake reaches the largest in April,shifting to negative values in zone IV from June to August and in zone V in June and July.Moreover,the monthly net water exchange per unit width decreases from north to south,and the direction and magnitude of water exchange are found to depend on the hydraulic gradients between the lake and groundwater and the hydraulic conductivity of the slope collapse.

Hyporheic Zone Flow Disruption from Channel Linings： Implications for the Hydrology and Geochemistry of an Urban Stream, St. Louis, Missouri, USA

Cement channel linings in an urban stream in St. Louis, Missouri increase event water contributions during flooding, shorten transport times, and magnify geochemical variability on both short and seasonal timescales due to disruption of hyporheic flowpaths. Detailed analyses of water isotopes, major and trace elements, and in situ water quality data for an individual flood event reveal that baseflow contributions rise by 8% only 320 m downstream of the point where this particular channel changes from cement-lined to unlined. However, additional hydrograph separations indicate baseflow contributions are variable and can be much higher（average baseflow increase is 16%）. Stream electrical conductivity（EC） and solute concentrations in the lined reach were up to 25% lower during peak flow than in the unlined channel, indicating a greater event flow fraction. In contrast, during low flow, stream EC and solute concentrations in the lined reach were up to 30% higher due to the restricted inflow of more dilute groundwater. Over longer timescales, EC, solute concentrations, turbidity, and bacterial loads decrease downstream signifying increasing contributions of dilute baseflow. The decreased connectivity of surface waters and groundwaters along the hyporheic zone in lined channels increases the hydrologic and geochemical variability of urban streams.

Determination of seepage velocity in streambed using temperature record of Russian River, USA

The vertical seepage velocity is an important parameter in the groundwater-surface water （GW-SW） exchange process. It is reported that the periodical fluctuated temperature record of the streambed can be used to determine the seepage velocity. Based on a 1-D flow and heat transport model with a sinusoidal temperature oscillation at the upstream boundary, a new analytical model is built. This analytical model can be used to determine the seepage velocity from the amplitude ratio of the deep and shallow test points. The process of calculation is discussed. The field data are superimposed by multi-periods, so the spectrum analysis and the data filtering are desirable. For the typical seepage medium, the analytical model is effective to compute the seepage velocity between -2 m/d and 6 m/d by using the record of the daily period fluctuation. The temperature time-series analytical model is used to determine the upwards seepage under the condition that the spacing of test points is small （less than 0.2 m）. Lastly, a case study for the Russian River shows that this model is very convenient to determine the temporal changes of the GW-SW exchange.