16 research outputs found
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Direct Observation of the Depth of Active Groundwater Circulation in an Alpine Watershed
The depth of active groundwater circulation is a fundamental control on stream flows and chemistry in mountain watersheds, yet it remains challenging to characterize and is rarely well constrained. We collected hydraulic conductivity, hydraulic head, temperature, chemical, noble gas, and 3H/3He groundwater age data from discrete levels in two boreholes 46 and 81 m deep in an alpine watershed, in combination with chemical and age data from shallow groundwater discharge, to discern groundwater flow rates at different depths and directly observe active and inactive groundwater. Vertical head gradients are steep (average of 0.4) and thermal profiles are consistent with typical linear conductive continental geotherms. Groundwater deeper than ∼20 m is distinct from shallow groundwater and creek water in its chemistry, noble gas signature, and age (dominantly >65 years compared to <9 years). Together these results suggest low vertical groundwater flow velocities and a relatively shallow active circulation depth of ∼20 m. This hypothesis is tested with a simple 2-D numerical fluid flow and heat transport model representing a hillslope transect through the two boreholes. The modeling indicates that the subhorizontally bedded sedimentary rocks underlying the basin are highly anisotropic with low vertical hydraulic conductivity, and at most ∼10% of bedrock recharge (equivalent to <2% of stream baseflow) flows below a depth of 20 m. The study demonstrates the considerable value of discrete-depth hydrogeologic, chemical, and age data for determining active circulation depth, and illustrates an approach for maximizing the utility of individual boreholes drilled for mountain bedrock aquifer characterization
Environmental effects on the aquatic system and metal discharge to the Mediterranean Sea from a near-neutral zinc-ferrous sulfate mine drainage
After mine closure in the 1980s and
subsequent shutdown of the dewatering system,
groundwater rebound led to drainage outflow from
the Casargiu gallery (Montevecchio mine, SW
Sardinia, Italy) beginning in 1997. Mine drainage
had pH 6.0 and dissolved concentrations of sulfate
(5000 mg/L) and metals (e.g., 1000 mg/L Zn,
230 mg/L Fe, 150 mg/L Mn) much higher than those
previously measured in groundwater under dewatering
conditions. As compared with the first stages of rebound
at Casargiu, a very high contamination level still persists
after more than 15 years of flushing. Mine drainage (20–
70 L/s; pH 6.0±0.2; Zn-Mg-Ca-SO4 composition)
flowed into the Rio Irvi. Abundant precipitation of
amorphous Fe(III)-(oxy)hydroxides occurred.
Moreover, sulfate-bearing green rust was observed to
flocculate in the reach of the Rio Irvi where pH was still
circumneutral. Water sampling along this stream for
about 6 km almost to its mouth in the Mediterranean
Sea showed a pH decrease from 6.0 to 4.0 and a
significant removal of Fe (46 %) and As (96 %),
while sulfate, Zn, Mn, Co, Ni, and Cd showed small
variations downstream. Lead was initially adsorbed onto
Fe(III)-(oxy)hydroxides, then desorbed as pH dropped
below 5. The estimated amount of dissolved metals
discharged into the Mediterranean Sea is significant
(e.g., 900 kg/day Zn, 1.4 kg/day Cd, 5 kg/day Ni). In
particular, a conservative estimation of the amount of Zn
discharged to the sea is about 330 ton/year, which would
correspond to 1.4 % of the global annual flux of dissolved
Zn from uncontaminated rivers to the oceans
Hydrozincite seasonal precipitation at Naracauli (Sardinia-Italy): hydrochemical factors and morphological features of the biomineralization process
Hydrozincite [Zn5(CO3)2(OH)6] precipitation from Naracauli waters (SW Sardinia) is promoted by a microbial community made up of a filamentous cyanobacterium (Scytonema sp.) and a microalgae (Chlorella sp.). Hydrozincite bioprecipitation is responsible for the natural removal of heavy metals, especially Zn, from the stream waters. Thus, hydrozincite could be used to attenuate metal pollution in mining waters. Information on environmental conditions that promote the biomineralization process is fundamental for the development of remediation strategies. This paper aims to investigate the variables controlling the biomineralization process, and the hydrochemical factors that affect hydrozincite precipitation. Morphological analysis shows that hydrozincite morphology varies, and depends on the environmental conditions. Changes were observed between samples collected in late spring and samples collected in summer, and among samples precipitated under different water flow conditions. According to field observations, correlated with speciation and equilibrium calculation, the maximum intensity in hydrozincite precipitation occurs in late spring of rainy years, when the hydraulic regime in the stream reaches stationary conditions, and SI values with respect to hydrozincite reach the highest values. Concomitantly, Zn2+/CO32- molar ratio reaches values close to 1, indicating that kinetic processes have a role on the hydrozincite biomineralization process
Quantifying biomineralization of zinc in the rio naracauli (Sardinia, Italy), using a tracer injection and synoptic sampling
Streams draining mined areas throughout the world commonly have high concentrations of Zn. Because Zn is not easily removed from stream water and because it can be toxic to aquatic organisms, its presence is a persistent
problem. The discovery of biomineralization of Zn-bearing solids in the mine drainage of Rio Naracauli, in Sardinia, Italy, provides insights into strategies for removing Zn and improving water quality in streams affected by mine drainage. Until now, the transport and attenuation of Zn has not been quantified in this stream setting. A continuous tracer injection experiment was conducted to quantify the biomineralization process and to identify the loading of constituents that causes a change from precipitation of hydrozincite [Zn5(CO3)2(OH)6] in the upstream reach to precipitation of a Zn-silicate phase downstream. Based on the mass-load calculations derived
from the tracer experiment, about 1.2 kg/day of Zn is sequestered in hydrozincite. This biomineralization represents nearly 90% removal of Zn. Other elements such as Pb and Cd also are sequestered, either in the hydrozincite, or in a separate phase that forms simultaneously. In the lower 600 m of the stream, where the Zn-silicate forms,
as much as 0.7 kg/day Zn are sequestered in this solid, but additions of Zn to the stream from groundwater discharge lead to an overall increase in load in that portion of the Rio Naracauli