A Combined Riverbank Filtration and Aquifer Storage and Recovery System to Address Temporal Water Scarcity

Transcription

1 ABSTRACT & POWERPOINT PRESENTATION A Combined Riverbank Filtration and Aquifer Storage and Recovery System to Address Temporal Water Scarcity Laxman Sharma Ph.D. Student, Department of Civil and Environmental Engineering University of Hawaii at Mañoa Honolulu, Hawaii Managed Aquifer Recharge Symposium January 25-26, 2011 Irvine, California Symposium Organizers: National Water Research Institute Orange County Water District Water Research Foundation

2 A combined River Bank Filtration and Aquifer Storage and Recovery system to address temporal water scarcity Laxman Sharma 1 and Chittaranjan Ray 2* 1 Department of Civil & Environmental Engineering, University of Hawaii at Manoa, Honolulu, HI Department of Civil & Environmental Engineering and Water Resources Research Center, University of Hawaii at Manoa, Honolulu, HI Riverbank filtration (RBF) is an accepted method of obtaining drinking water from rivers in which wells placed at a distance from the river induce surface water to pass through the aquifer as they are pumped. The quality improvement of the filtrate water, as it is pumped out of the production well, is known to be significant compared to the source water. Seasonal variations in river discharges are common in the world and some areas face severe water shortages or use restrictions in the low flow seasons impacting economic growth. This scarcity can be mitigated by an innovative combination of RBF and aquifer storage and recovery (ASR) systems. RBF can be used to tap rivers when there is excess flow and store the drawn water into an ASR system to retrieve when required. This approach has some distinct advantages over recharge basins. The evaporation losses are greatly reduced and it does not require a large surface area for recharge. There are several quality concerns when RBF and ASR systems are run in series. The quality of the filtrate must comply with local regulations before injection into a potable aquifer. During the cyclical process of injection of water, storage within the aquifer, and subsequent withdrawal, various biogeochemical reactions can occur altering the quality of stored water. If dissolved organic carbon is present in the injected water, microbial reactions will degrade carbon and deplete the oxygen in the aquifer and make the environment more reduced. This may cause denitrification as well as the dissolution of redox sensitive species such as iron, manganese, sulfates, etc. A combination of RBF and ASR is considered for the Albany region of Georgia (USA) where seasonal shortfall in water availability in the Flint River occurs at one time and excess runoff occurs at another time. The simulated RBF well draws water from the Flint River through the Upper Floridian Aquifer and it is injected into the deeper Clayton Aquifer for storage and subsequent recovery. Numerical modeling studies using MODFLOW and geochemical modeling with multi-component reactions using PHT3D were carried out to examine the fate of the extracted water through RBF and fed into an ASR system in limestone aquifers, using publicly available data. This study tests the conceptual framework of having a combined system of RBF and ASR schemes, and more importantly, looks at the hydrogeochemical changes that are likely to occur. This presentation reports the findings of the study. * Corresponding author. cray@hawaii.edu

3 Managed Aquifer Recharge Symposium Jan 25-26, 2011 Atrium Hotel, Irvine, Orange County, CA A Combined Riverbank Filtration and Aquifer Storage and Recovery System to Address Temporal Water Scarcity Laxman Sharma and Chittaranjan Ray Department of Civil & Environmental Engineering and Water Resources Research Center University of Hawaii at Manoa

4 Outline Introduction Why RBF-ASR? Quality considerations through modeling Conclusions 2

5 evelopingnewwaterresources.aspx 3

6 RBF Wells along the Rhine River

7 Ganges - India

8 Sabarmati River, Ahmedabad, India

9 Flint River, Albany, Georgia, US

10 Strategy to Meet Water Demand Water demand ever increasing. Surface storage ET losses, or area constraints. Water banking - inject potable water to an aquifer during periods of excess and withdraw when required! In flashy rivers, shallow detention basins with RBF wells and galleries can be adopted.

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12 Why RBF-ASR?? June to September/October high irrigation pumpage Flint River and the Floridian Aquifer are hydraulically connected Reduction in base flow in river negative downstream impacts Farmers have been asked not to pump and been compensated for not irrigating in summer Rise in population and expansion of existing industries already used up the maximum allocation from the Floridian Aquifer (no new wells can be installed).

13 Modeling Study near Albany, GA RBF Aquifer: The Ocala Limestone (Upper Floridan Aquifer), partially exposed,130 ft thick, K~ 450 ft/d. Clayton Aquifer is the ASR aquifer, located at ft below ground surface and K ~ 66 ft/d,. K values from estimated from transmissivity values.

14 RBF-ASR setup The Flint Rivers cuts into the surficial Ocala aquifer(upper Floridian); the RBF wells are screened in this aquifer. The filtrate is injected into the deeper Clayton confined aquifer. Strategy RBF wells to operate 6 months (simultaneous injection) and ASR wells to operate for the remaining 6 months to meet summer demand. Another strategy pump for 6 months (2Q) and recover over the year (Q) not modeled.

15 Operating Rules:Pump Rates/m Start time (days) End time (days) RBF well (GPD) ASR well (GPD) remarks start up period to storage to ext. supply to storage to ext. supply RBF: October to March extract ASR : October to March inject to storage April to September extract for use 13

16 RBF Model setup Pumping wells located at 120 feet from the river. Total pumpage 10 MGD with 10 wells, 200 ft c/c. A preliminary 3-D numerical model prepared and run with MODFLOW. Used this to make a more detailed computationally efficient 2-D model of the vertical section across the river and riverbank. River data: Station ID (Flint River at Albany) USGS Upper Floridian Aquifer Well ID 12K129 Clayton Aquifer Well ID Well ID 12L020 14

17 RBF Model setup Modeled using MODFLOW and PHT3D (MT3DMS+PhreeqC) for general equilibrium reactions, and calcite dissolution, organic carbon redox reactions, a preliminary look at possibility of Arsenic leaching. 15

18 RBF modeling RBF Model Domain: ft deep with 14 layers, 520 ft long. Vertical resolution at the river section at the top left corner refined to capture river stages. Time variant flow depicted in the river cells. General head boundary (GHB) set at the right boundary, 154 ft at a distance of 6000 ft.

19 ASR Model setup Total pumpage 10 MGD with 10 wells, 350 ft c/c. A 2-D vertical model, one half of the symmetrical area modeled. ASR well is at the left boundary and a GHB with specified ambient concentrations at 900 ft (fixed from iteration). Grid spacing closer near the well. Operation rule of injecting and extraction is imposed as the left boundary condition along with the chemistry of the injecting water. 17

20 ASR Well Obs Well ASR Modeling setup Obs Well Confined Aquifer (Clayton Aquifer) GHB Rectangular grid for the ASR modeling, with greater resolution near the ASR well 900 ft Model Domain: 125 ft thick (625 to 750 ft bgs) 5 layers, 2-D symmetrical model, grid interval closer near the well as that is where the action is! Left Boundary ASR well, Right boundary GHB, fixed at 900 ft iteratively where GW chemistry remained relatively unchanged.

21 Aqueous components as model inputs from charge balanced data set. Aqueous Component Flint River (mol/l) Upper Floridian Aquifer (mol/l) Lower Floridian Aquifer (mol/l) DOC Temperature Dissolved Oxy. 4.4e-4 to 6.9e E-04 0 Ca(+2) 5.24E E E-04 Mg(+2) 5.76E E E-04 Na(+) 2.94E E E-03 K(+) 4.60E E E-05 Fe(+3) 1.38E E E-06 Mn(+3) 7.81E E E-08 Si 1.09E E E-04 Cl(-) 7.48E E E-03 C(+4) 5.98E E E-03 S(+6) 9.68E E E-04 N(+5) 2.71E E E-06 N(+3) 2.14E E-08 0 Amm (+) 1.61E ph Units: mol/l except Temp ( O C) and ph is dimensionless. 19

22 Adding on Reaction Network Reaction Stochiometry Aerobic CH 2 O + O 2 CO 2 + H 2 O degradation Denitrification CH 2 O NO 3 HCO H 2 O N H + Iron (3) FeOOH + CH 2 O Fe 2+ + HCO 3 + H + reduction Sulfate 2CH 2 O+ SO 2 4 2HCO 3 + H 2 S reduction Pyrite oxidation FeS O H 2 O Fe(OH) 3 + 2SO H + Reactions are modeled as Partial Equilibrium Approach (McNab and Narasimhan, 1994) Acidity generated by the presence of CO 2 or the excess protons are expected to cause calcite dissolution CO 2 + H 2 O + CaC0 3 2HCO 3 + Ca 2+

23 Results and Discussion Organic carbon degradation is the primary process that drives the geo-chemical processes in the aquifer. Anoxic conditions develop for most of the time. The RBF water is pumped into the deeper aquifer without any treatment (for the modeled case). The process is mostly anoxic. 21

24 days days days days Simulated time series plot of concentrations at the RBF pumping well (First 50 days is spin-up time and simulated additionally for 2 years. Pump is off during the periods and days.) 22

25 Simulated time series plot of concentrations at the RBF pumping well At 200 days, aerobic reaction ceases and denitrification started. DOC picks up, 230 days N(5) exhausted, then Fe(+3) exhausted, reducing situation, pe reversal, partial reduction of sulfate.

26 ASR discussion During the injection periods, the plots show the concentration of the injectant. When extraction occurs, 230 to 410 days and 590 to 770 days, the, plots show increased activities. Denitrification, iron reduction and some sulfate reduction occur during pumping. repeated flow reversals facilitate dissolution environments at the vicinity of the reversible wells, which can offset well clogging. 24

27 days days days days Simulated concentration time series at the ASR well. (First 50 days is spin-up time and simulated additionally for 2 years. 25

28 Simulated concentration time series at the ASR well. (First 50 days is spin-up time and simulated additionally for 2 years. Complete oxygen consumption, denitrification, Fe reduction and even e sulfate reduction for a very brief moment occurs.) 26

29 Possibility of Arsenic leaching Another set of simulation run of ASR including oxidation of pyrite was also considered. The RBF water was injected into this arseno-pyrite bearing (ratio of arsenopyrite to the pyrite assumed to be 1 in 1000). Redox modeling showed that the maximum amount of As(3) and As(5) were negligible 5.27 E- 11 mol/l and 9.13 E-11 mol/l At a narrow band around ph 7, As(3) and As(5) adsorb strongly to iron oxides, clays and silicates or onto iron (hydro)oxides or sulfides. 27

30 days days days days Concentration time series plot at the ASR well for the second case when pyrite oxidation is considered. 28

31 Concentration time series plot at the ASR well for the second case when pyrite oxidation is considered. ph averages near neutral, sulfate reduction initially takes place e but when the pump is extracting water at 230 days, pyrite dissolution should be taking place as the concentrations of sulfates and arsenics pick up then. This stops when the ASR well starts injecting. 29

32 Conclusion Coupled RBF and ASR is an attractive option to address water scarcity in drier months of the year. RBF can be used to abstract water of better quality and store it through ASR and reuse it when required. The processes taking place during RBF withdrawal; ASR injection, storage and extraction can be numerically modeled. Transport and fate of arsenic and heavy metals is an immediate concern for RBF and ASR schemes and a more detailed description of in-situ parameters and elucidation with field studies and modeling is required. The RBF ASR coupling can be easily adjusted to incorporate simple tweaks to water quality before pumping in for storage. 30

33 I am looking for data sets to model 31