Sources of dissolved oxygen in monitoring and ...

Sources of dissolved oxygen in monitoring and pumping wells

Matthijs Bonte, Bas Wols, Kees Maas & Pieter Stuyfzand

Hydrogeology Journal Official Journal of the International Association of Hydrogeologists ISSN 1431-2174 Hydrogeol J DOI 10.1007/s10040-016-1477-9

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Author's personal copy Hydrogeol J DOI 10.1007/s10040-016-1477-9

PAPER

Sources of dissolved oxygen in monitoring and pumping wells Matthijs Bonte 1,2 & Bas Wols 1 & Kees Maas 3 & Pieter Stuyfzand 1,4

Received: 24 February 2016 / Accepted: 16 September 2016 # Springer-Verlag Berlin Heidelberg 2016

Abstract Groundwater monitoring and pumping wells set in anoxic aquifers require attention to keep the groundwater free of dissolved oxygen (DO). In properly constructed monitoring or pumping wells, two processes can however still introduce oxygen to anoxic groundwater: (1) permeation of oxygen through polymer materials such as silicone, PVC, HDPE or Teflon, and (2) thermally driven convection, which can occur in all types of piezometers or wells, regardless of construction material, when the water table or pressure head is close (30 m) investigated here in which oxygen intrusion was observed only up to around 15 m depth. In the heat balance, on all boundaries a temperature is prescribed which is based on field measurements. At the bottom and surface, a fixed temperature is used, and at the sides a temperature profile is used. Simulations were performed with and without permeation of oxygen through the wall of the monitoring well, while thermal convection and diffusion in the water were always included. For the simulations without permeation, a zero oxygen concentration was used for the surrounding groundwater. For the simulation with permeation, a thin diffusive barrier is modelled at the sides that allow (diffusive) permeation of oxygen from the groundwater. The transport of oxygen through the diffusive barrier is dependent on the difference in oxygen concentration between water in and outside the well, the permeation coefficient (K = 0.04 × 10−10 m2/s, value for PVC in Table 1), and thickness of diffusion layer, which is set at 2 mm, equal to the wall thickness. The oxygen concentration outside the well is a step profile with zero at the bottom and

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1 at the top (Fig. 2). At the bottom, a no flux boundary of oxygen is prescribed. Mass transfer from the atmosphere occurs at the free water surface. According to liquid film theory, the mass transfer of oxygen from the atmosphere to the water can be written as the following flux N: N¼

DW ðcw −ca H Þ zW

ð13Þ

where Dw is the diffusion coefficient of dissolved oxygen, cw the concentration of dissolved oxygen in the water, ca the concentration of oxygen in the air and H is Henry’s constant. H¼

RT kH

ð14Þ

The constant kH is temperature dependent and can be written according to the Van’t Hoff equation:





1 1 k H ¼ k H;re f exp −C ð15Þ − T T re f The maximum concentration of dissolved oxygen in the water will then be equal to (stationary solution as N = 0): cw ¼ ca H

ð16Þ

It is possible to use the equation for the flux N as a boundary condition in the model. However, the liquid film thickness is an important parameter for the flux at the free surface and needs to be estimated; therefore, in this work, at the surface a (normalized) oxygen concentration of 1 is used. The normalized oxygen concentration needs to be scaled with the maximum DO concentration as shown in Table 4. Simulations were performed using both approaches. In case a flux boundary was used, results converged for a liquid film thickness below 0.1 mm. In case a concentration boundary is used, similar results as for a flux boundary are obtained as long as the mesh size is relatively small ( 10 cm) wells, instabilities will develop at very small temperature differences. Analytical modelling of oxygen transport by permeation In order to assess whether permeation could also result in similar DO concentrations in the well riser, oxygen-depth profiles were calculated for various radii resulting from permeation of oxygen saturated groundwater (10 mg/L) surrounding the well to 15 m BSL (Fig. 5). Note that actual field data showed that these high concentrations were not present in groundwater outside the riser, so that these calculations are done to assess whether permeation could also contribute to oxygenation under different field conditions. Results are shown for both PVC and HDPE, the two most used plastic types for both monitoring and production wells. The results indicate that oxygen permeation can oxygenate water

Fig. 5 Calculated dissolved oxygen (DO) depth profiles for PVC and HDPE wells at t = 1–1,000 days with initially no oxygen present, placed in groundwater with an abrupt transition from oxic water (10 mg/L) to anoxic water (0 mg/L) at 15 m BLS

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contained in monitoring wells. For both PVC and HDPE 1.5 and 2″ monitoring wells, within 1 year, 100 % saturation of water in the standpipe is reached in the depth section where the well is surrounded by oxic groundwater. For larger diameter wells, the effect of permeation is much slower. For a PVC 10″ well, water in the standpipe is expected to reach around 3 mg/L DO after 1,000 days, while for a HDPE well this is around 8 mg/L. Transport of oxygen by diffusion in the water itself along the depth of the standpipe is nearly negligible within this timeframe resulting in a very sharp interface at 15 m BLS. This process has not yet been reported in the literature, and field and experimental data are needed to confirm that permeation can indeed introduce oxygen into wells. Oxygen transport modelled with CFD for summer and winter temperature profiles The proof of principle modelling confirms that the model is capable of predicting convection cells (Fig. 6). Cells that are roughly equally sized in length and width occur over the depth of the well, transporting the colder water (visualized in red)

Fig. 6 Proof of principle for numerical modelling: a the water velocities, ranging from 0 (blue) to 0.013 (red) m/s, b the water temperature, ranging from 2 °C (red) to 11 °C (yellow/white), and c the streamlines

downward. In the case of a higher temperature at the top, no convection cells developed. Imposing a typical winter temperature profile as observed on 15 March 2012, results in convection cells with a maximum rotation velocity of 0.009 m/s (Fig. 7). The cells are not as clearly visualized as in the proof of principle, due to the coarser mesh and the stretching of the x-axis in order to show the complete well. The convection cells occur over the full depth of 10 m where the negative temperature gradient is applied. At lower depths, the temperature profile has a positive gradient and no cells develop. Figure 8 shows the simulated temperature and DO versus depth profiles for a number of time steps. In these simulation results for the winter temperature profile, relatively large temperature and DO fluctuations occur which are not observed in the field data (compare Figs. 3 and 8). This is likely a numerical artefact caused by the relatively coarse mesh that is used for this simulation. Also, the fluctuations shown in the simulated profiles occur over small depths (cm-range), while the field measurements had a depth interval of 1 m. The convection cells transport oxygen from the top boundary downward

Fig. 7 a Velocity (vertical direction), b temperature, and c oxygen in the well without permeation (high DO profile, 15-3-2012) after 7 days of simulation using a realistic temperature profile. The x-direction is stretched out 40 times. The depth of the well (L) is 30 m, of which the upper 15 m were simulated and the upper 10 m are shown here. The coordinate z represents the vertical position in the well (L – z = 30 m represents the top of the well)

Author's personal copy Hydrogeol J Fig. 8 Modelled a temperature profile and b oxygen profile at several time steps without permeation (high DO profile, 153-2012) using a realistic temperature profile

into the well. The oxygen penetrates down to a depth of about 10 m, corresponding to the depth of the convection cells. Simulation of a summer temperature profile (based on 216-2011 data) using a mesh similar to scenario 1, showed that no convection cells develop (results in Figure S2 of the ESM). As a result, no temperature variations occur (Figure S3 of the ESM). In the simulated 3 weeks, no oxygen is transported into the well and only at the top of the well is an increase in oxygen observed resulting from diffusion.

pumping pipe and will never go down to 0.0 mg/L due to the combined effect of oxygen diffusion from air into the water and convective mixing. If the extraction pump is to discharge at a higher rate, the amount of oxygen transported downward will be diluted by the anoxic water directly transported from the filter. These findings highlight the restrictions for low-flow purging in shallow wells during winter where a thermally instable situation can occur above the filter. The long tailing of oxygen-enriched water simulated here is in

CFD of a winter temperature profile under groundwater sampling This scenario was simulated with half of the domain (using a symmetrical middle plane) to avoid numerical convergence problems. The simulation using a winter temperature profile with groundwater sampling at a depth of 25.1 m showed small convection velocities above the pumping pipe (Fig. 9). The pumping in combination with a negative temperature profile induces one large convection cell stretching over the complete height of the domain. Note that due to the contrast in velocity between pumping and thermal convection, the convection cell is not clearly visible in Fig. 9. This large convection cell is clearly different from the smaller ones that developed without pumping; however, the velocities in the convection cell remain small in comparison with the velocities induced by the pumping. As a result of the convection cell, the oxygen concentration above the pumping pipe is slowly reduced, slowly leaking oxygen to the discharge pipe, and after 2 h it is almost completely replaced by anoxic water (Fig. 10). The simulated oxygen concentration in extracted water during winter is around 0.2 mg/L, while a similar simulation in summer showed concentrations to rapidly decrease to 0.0 mg/L. The elevated oxygen concentrations remain for a few hours during pumping. The length of this time period will depend on the volume of oxygen-rich water above the

Fig. 9 Effect of pumping (100 ml/min) on a velocity, b temperature and c oxygen profiles in a part of the well. The x-direction is stretched out 4 times

Author's personal copy Hydrogeol J Fig. 10 a Oxygen concentration through the pumping pipe as a function of time; b Zooms in on the time period just after pumping commenced. Pumping started at t=2 h

agreement with the field data presented by Vroblesky et al. (2007) who measured DO concentrations around 1 mg/L with a purging flow of 200 ml/min.

Implications of oxygen intrusion In case of an anoxic aquifer with groundwater containing dissolved iron and manganese, introduction of oxygen can result in oxidation of dissolved Fe2+ and Mn2+ to much less soluble Fe3+ and Mn4+ and subsequent precipitation of iron- and manganese-(hydr)oxides (Bustos Medina et al. 2013). As such, the intrusion of oxygen may have implications for sampling of groundwater (Puls et al. 1992; Puls and Powell 1992; Roy and Fouillac 2004) and may even result in well clogging (Houben and Trestakis 2007; van Beek 2010). Our simulations show that thermal convection may introduce DO which can influence the collected groundwater sample if well purging is insufficient (e.g. using no-purge or micro-purging) and the convective flux reaches a greater depth than the intake of the sampling device. If the thermal convection cells reach the (top of) shallow well screens, then well-screen clogging may take place. In our case, oxygen intrusion was observed to around 12 m depth. Oxygen intrusion can also influence other water-quality aspects, as the precipitated iron- and manganese-(hydr)oxides may provide sorption sites (Appelo and Postma 2005; Dzombak and Morel 1990) which can lower concentrations of trace elements in a well compared to the aquifer, especially when well purging is insufficient. Well clogging by precipitating iron (hydr)oxide precipitates is a very common type of chemical well clogging. The dominant mechanism is the mixing of oxic and anoxic groundwater in the well (Houben 2003a, b). Thermal convection can provide an additional mechanism for chemical clogging to occur if: (1) the aquifer contains groundwater with high Fe2+ concentrations, (2) the submersible pump is set above the maximum intrusion depth of oxygen (which was 12 m BSL) and (3) the well is regularly switched off and on. Every time the well is switched on, the water in the standpipe below the pump is refreshed with anoxic groundwater with

ferrous iron. When the pump is switched off, thermal convection during fall and winter will transport oxygen downward and this will react with the ferrous iron which will whirl down the well and be deposited in the wells’ sump or stick into the filter slots. This hypothesized process is confirmed by camera inspections (Bustos Medina et al. (2013) and anecdotal evidence by van Beek (2010) that chemical well clogging progresses more rapidly when wells are used intermittently. A practical way of preventing this clogging process is to position the submersible pump at sufficient depth, below the maximum intrusion depth of oxygen, taking into account the drawdown in the well during pumping. The other way round, the introduction of oxygen from above can be accompanied by CO2 losses at the top via rising convection cells and degassing, which may contribute to the precipitation of CaCO3 from initially saturated groundwater, thereby forming CaCO3 encrustations which may contribute to well clogging. Permeation can potentially cause clogging if a production well screen taps an anoxic aquifer, but the riser is set through an oxic aquifer. The build-up of oxygen in the production well is however quite slow, especially in large-diameter production wells which have a considerable wall thickness and large water volume (Fig. 5). In PVC wells, the permeation velocity is even smaller indicating that permeation is unlikely to contribute greatly to chemical well clogging.

Conclusions Modelling suggests that both permeation and convection can introduce oxygen into anoxic wells. Temperature and oxygen field data show that convection is primarily responsible for the monitoring well that was investigated in this study. Analytical modelling suggests that permeation could be another contributor to oxygen, especially in small-diameter HDPE wells if the riser is traversing an oxic aquifer, but less so in PVC wells and large diameter wells. Note that field and experimental data are needed to confirm that permeation can indeed introduce oxygen into wells.

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Oxygen transport due to thermal convection is active in all types of wells with a relatively shallow water table (