E F F E C T OF W A T E R D E P T H IN GROUNDWATER

RECHARGE BASINS ON INFILTRATION

By Herman Bouwer,1 Member, ASCE, and R. C. Rice2

ABSTRACT: Sediment or organic clogging layers on the wetted perimeter are the rule rather than the exception for infiltration basins for artificial recharge of groundwater. When the water depth in such basins is increased, the clogging layer is compressed. This causes infiltration rates to increase much less than expected from the water depth increase alone, and rates may actually decrease. Such de­creases are especially severe where the decreased turnover rate (increased detention time) of the water in the infiltration basins due to increasing the water depth leads to increased growth of suspended algae. These algae form a filter cake on the basin bottom. Also, because of photosynthesis, these algae increase the pH of the water, which can cause precipitation of calcium carbonate. This precipitate and the algal filter cake both aggravate the clogging problem and cause infiltration rates to de­crease even further. Consolidation theory is used to explain the compression of the clogging layer. The conclusions are supported by field and laboratory studies. Careful analysis of the situation and on-site experimentation are needed to determine the optimum water depth for recharge basins.

INTRODUCTION

Where groundwater is artificially recharged via infiltration basins, it is often desirable to maximize the hydraulic capacity of the basins to get as much water into the ground as possible. The hydraulic capacity usually is expressed as the hydraulic loading rate, which is the accumulated infiltration over a long period that includes the times that the basins are dry for infil­tration recovery (including sediment removal or other cleaning). For year-round recharge systems, hydraulic loading rates typically vary from about 30 m/yr to 300 m/yr , depending on hydraulic conductivity of the soils, groundwater levels, quality of water going into the basins, climate, and rec­reational or environmental constraints. Maximum hydraulic loading rates for a given site can be achieved with proper combinations of flooding and drying periods and cleaning of the basins, pretreatment (desilting) of the water, chemical additives like calcium to increase the hydraulic conductivity of clay accumulations on the basin wetted perimeter, proper selection of infiltration systems (i.e., basins with essentially stagnant water or channels with flowing water), and proper selection of water depth in the basins or channels. The latter is the subject of this paper.

EFFECTS OF WATER DEPTH AND CLOGGING

How water depth in an infiltration basin or channel affects the infiltration

'Dir., U.S. Water Conservation Lab., 4331 E. Broadway Rd., Phoenix, AZ 85040. 2Agr. Engr., U.S. Water Conservation Lab., 4331 E. Broadway Rd., Phoenix,

AZ. Note. Discussion open until January 1, 1990. To extend the closing date one month,

a written request must be filed with the ASCE Manager of Journals. The manuscript for this paper was submitted for review and possible publication on May 24, 1988. This paper is part of the Journal of Irrigation and Drainage Engineering, Vol. 115, No. 4, August, 1989. ©ASCE, ISSN 0733-9437/89/0004-0556/$1.00 + $.15 per page. Paper No. 23737.

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/ V WATER — TABLE

FIG. 1. Schematic of Wetted Zone below Clean Recharge Basin with Deep Groundwater Table

rate depends on several conditions. If, for example, the basin is "clean" (wetted perimeter not covered by clogging material) and large enough so that bank infiltration is small compared to bottom infiltration, and the groundwater table is deep (Fig. 1), increasing the water depth in the basin will produce only a small increase in seepage. This can be demonstrated by applying the Green-Ampt equation (Bouwer 1978) to the wetted zone be­tween the bottom of the basin and the groundwater table. On the other hand, if the groundwater table is so high that it is only a small distance below the water level in the basin (Fig. 2), there is an essentially linear relation be­tween height of water surface in basin above groundwater table some dis­tance away from the basin, and infiltration rate (Bouwer 1978), regardless of whether the basin is clean or has a clogging layer on the wetted perimeter. Increasing the water depth in the basin will then produce a substantial in­crease in infiltration rate.

Most recharge basins have some kind of clogging material on their wetted perimeter. This clogging can be caused by accumulation of silt, clay, or other fine material that was suspended in the water entering the recharge basin, by algae growth on the bottom or in the water (the unicellular types like Carteria klebsii can form particularly troublesome clogging layers or

WATER TABLE

FIG. 2. Schematic of Clean Recharge Basin with Relatively High Groundwater Table

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TABLE

FIG. 3. Schematic of Recharge Basin with Clogging Layer on Wetted Perimeter and Unsaturated Flow to Groundwater

"filter cakes" on the basin bottom and banks; see Bouwer and Rice 1984), by precipitation of calcium carbonate in the water due to increases in the pH caused by algal activity, and by biological activity on the wetted perim­eter (growth of bacteria and other microorganisms, production of polysac­charides and other biopolymers, etc.). The hydraulic conductivity of the clogging layer usually is so low that this layer controls the infiltration rate. If the groundwater table is more than about 1 m below the bottom of the infiltration basin, the zone between the basin bottom and the groundwater table then is unsaturated, assuming, of course, that the vadose zone is uni­form and that there are no restricting layers that can form perched ground­water. The water in this unsaturated zone will have negative pressure heads and it will move essentially vertically downward due to gravity and, hence, at unit hydraulic gradient (Fig. 3). In these infiltration systems, the hydraulic head due to water depth is completely dissipated through the clogging layer. The negative pressure head in the unsaturated material and, hence, at the interface between the clogging layer and the underlying soil, is controlled by the relation between unsaturated hydraulic conductivity and (negative) pressure head of the underlying soil (Bouwer 1982). Applying Darcy's equa­tion to the flow through the clogging layer (Bouwer 1982) then gives an essentially linear relation between water depth in the basin and infiltration rate.

However, as the water depth is increased, the clogging layer is compressed and becomes less permeable. Because of this, the increase in infiltration rate may only be moderate and there could even be a decrease. Moreover, if the infiltration rate does not increase in direct proportion to increasing the water depth, the rate of turnover of the water in the basin decreases and secondary effects such as increased algae growth and precipitation of calcium carbonate can aggravate the clogging process and reduce infiltration rates. In this pa­per, compression of the clogging layer due to increasing the water depth is analyzed with soil mechanics consolidation theory. Secondary effects are also discussed. The conclusions are illustrated with results from field and laboratory studies. The need for on-site experimentation to evaluate the ac­tual effect of water depth on infiltration rates and to determine the most desirable water depth for a given recharge basin, is stressed.

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COMPRESSION OF CLOGGING LAYERS DUE TO INCREASING

WATER DEPTH

In soil mechanics, consolidation theory is based on the intergranular pres­sure P,. This is the pressure transmitted between soil particles at their contact points. The intergranular pressure is calculated as the difference between total pressure P, and the hydraulic pressure Ph (Bouwer 1978). Compression of soil material occurs when P, increases (Bouwer 1978). The value of P, increase due to an increase in P, (e.g., by increasing the loading on the soil), or to a decrease in P,, (e.g., by lowering the groundwater table). According to elastic consolidation theory (Bouwer 1978), the vertical compression of a certain soil layer is calculated as

z = AP,~ (1) E

where z = vertical compression of the layer; L = thickness of layer prior to compression; AP, = increase in P,; and E = modulus of elasticity. Thus, after compression, the thickness of the layer is L - z.

The modulus of elasticity is determined from consolidation tests, where compression is measured in response to increasing the vertical loading on a soil sample (Bouwer 1978). Values of E range from 1 to 5 kg/cm2 for or­ganic soils to 10,000 kg/cm2 for dense gravels and sands (Bouwer 1978). Loose sediment in infiltration basins thus can be expected to have low values of E and to be very compressible.

To calculate AP, in Eq. 1, values of P, and Ph typically are plotted hor­izontally as a function of depth to yield P, and Ph lines for some initial condition (water depth in infiltration basin, depth of groundwater table, etc.). Values of P, at any depth are then determined as the horizontal distance between these lines. The process is repeated for the next condition (other water depth in basin or other depth of groundwater table), values of Pt at various depths are again evaluated, and the changes in P,, or AP,, are de­termined. This procedure has been applied, for example, to predict land sub­sistence by groundwater depletion (Bouwer 1978).

In Fig. 4, the procedure is used to show the increase of P, in the clogging layer on the bottom of an infiltration basin due to increasing the water depth in the basin. The P,, lines show that Ph increases linearly with water depth in the basin until they reach the clogging layer of fine material. Values of Ph then decrease linearly with depth in the clogging layer itself to a negative pressure head at the bottom of the clogging layer. This pressure head is equal to the negative pressure head in the unsaturated zone below the clogging layer (Bouwer 1982). If this unsaturated zone is uniform, the pressure head in the unsaturated zone will be constant with depth, as indicated by the ver­tical Ph line in this zone. The P, line in the clogging layer starts at the Ph value at the top of the clogging layer. However, P, values in the clogging layer increase faster with depth than the P;, values in the water above it, because the solid particles in the clogging layer are heavier than the water they displace. Below the clogging layer, P, increases with depth at a smaller rate than in the clogging layer itself because the underlying material is un­saturated with part of the pores filled with air which, of course, is lighter than water. The horizontal difference between the Ph and the P, lines is P,, which in Fig. 4 is shown for the center of the clogging layer. The diagrams

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FIG. 4. Schematic of P, and Ph Lines for Shallow Basin (Left) and Deep Basin (Right) with Clogging Layer of Fine Material on Bottom

show that P, in the clogging layer is greater for the deep basin (right) than for the shallow basin (left). Thus, the increase in water depth in the basins caused />,- in the clogging layer to increase, which compresses this layer and reduces its hydraulic conductivity.

Fig. 4 shows that Ph in the unsaturated material below the clogging layer is larger (less negative) in the right system then in the left system. This is to account for the fact that raising the water level in the infiltration basin will initially increase the infiltration rate, which in turn will cause a higher hydraulic conductivity in the unsaturated zone and, hence, a higher water content and a higher (less negative) pressure head. As the clogging layer becomes compressed however, infiltration rates will go down and the Ph line in the unsaturated material of Fig. 4 right, will shift to the left as Ph de­creases (becomes more negative). This will further increase Pt in the clog­ging layer and will lead to more compression, etc. The clogging layer in Fig. 4 has been taken relatively thick to show the pressure relations. The procedure, however, is also applicable to thin layers, for example, layers of only a few mm thick.

Actual relations between Pt, void ratio or porosity, and hydraulic con­ductivity for a given material must be determined experimentally, using a consolidometer (Bouwer 1978). The device must be modified so that it can also be used as a permeameter to determine the hydraulic conductivity of the material at various stages of compression. Compression of soil materials due to increasing P, is essentially irreversible. A reduction of P, will give a slight rebound, but the material will not return to its original porosity or "looseness." Thus, if increasing the water depth in an infiltration basin gives a decrease in infiltration rate, lowering the water surface again will not return the infiltration rates to their original values. Instead, the basin should be dried, the clogging layer should be removed, and the basin should be filled with a small depth of water.

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EXAMPLES

The elastic consolidation theory will now be applied to a numerical ex­ample to compute the effect of increasing the water depth in a recharge basin on the thickness (compression) and void ratio of the clogging layer and to determine the resulting decrease in hydraulic conductivity and infiltration rates. The clogging layer of accumulated sediment on the bottom of a re­charge basin is assumed to be 10 cm thick and to have a porosity of 50% (void ratio of 1). The density of the sediment particles themselves is taken as 2.6 g/cm3. For the calculation of P,, 1 cm3 of the clogging layer thus weighs (0.5 X 2.6) + (0.5 X 1) = 1.8 g/cm3. Thus 10 cm of the layer will exert a P, of 18 g/cm2. Assuming that the soil below the clogging layer is unsaturated with a pressure head of —20 cm water, the value of Ph at the bottom of the clogging layer is -20 g/cm2. Taking the initial water depth in the basin above the top of the clogging layer as 100 cm, P, at the bottom of the clogging layer thus is 100 + 18 = 118 g/cm2. Since Ph at the bottom of the clogging layer is —20 g/cm2, P, at the bottom of the clogging layer is 118 - (-20) = 138 g/cm2. At the top of the clogging layer, P, = Ph -100 g/cm2, so that P, = 0. Next, the depth of water in the basin is assumed to increase to 400 cm above the clogging layer. This increases P, at the bottom of the clogging layer to 418 g/cm2, and P, to 438 g/cm2. Thus, the increase in water depth produces an increase in P, at the bottom of the clog­ging layer from 138 to 438 g/cm2, or 300 g/cm2. Since P, at the top of the clogging layer is always 0 regardless of water depth, the average P,-increase for the 10-cm clogging layer due to the increase in the water depth thus is 150 g/cm2. This is the value that will be used for AP, in calculating the compression of the clogging layer with Eq. 1. The average increase of P, in the clogging layer is seen to be one-half the increase in water pressure due to increasing the water depth.

To calculate the effect of this increase in P, on the compression of the clogging layer and the resultant decrease in hydraulic conductivity, two sce­narios will be considered: (1) One where the restricting layer is very com­pressible, such as a very soft clay or an organic material like peat or muck; and (2) one where the clogging layer is less compressible, such as a fine sand. For the first condition, E will be taken as 1 kg/cm2.

Substituting this value in Eq. 1, along with AP, = 150 g/cm2 = 0.15 kg/ cm2, and L = 10 cm yields z = 1.5 cm. This reduces the thickness of the clogging layer from 10 to 8.5 cm. At a void ratio of 1, the 10 cm thick clogging layer can be visualized as consisting of a 5 cm layer of solids and a 5 cm layer of voids. After compaction, the solids layer is still 5 cm but the voids layer has been reduced to 3.5 cm. This reduces the void ratio from 5/5 = 1 to 3.5/5 = 0.7. Taking experimentally determined relations be­tween void ratio and hydraulic conductivity such as figure 13 on page 45 in Terzaghi and Peck (1948), a reduction in void ratio from 1 to 0.7 in a clay soil would give a reduction in hydraulic conductivity K of about 80%. As­suming K of the restricting layer before compression to be 5 cm/day, K of the layer after compression would thus be 1 cm/day. The infiltration rate with the 100 cm water depth and K of the clogging layer equal to 5 cm/ day can be calculated by applying Darcy's equation through the clogging layer as 5 X [100 + 10 - (-20)]/10 = 60 cm/day (Bouwer 1982). After increasing the water depth to 400 cm and compaction of the clay layer to

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8.5 cm with a reduction in its hydraulic conductivity from 5 cm/day to 1 cm/day, the infiltration rate is 1 X (400 + 8.5 + 20)/8.5 = 50 cm/day, or slightly less than the infiltration of 60 cm/day for the 100-cm water depth.

Thus, increasing the water depth from 100 to 400 cm in this case did not produce an increase in infiltration rate, but even a slight decrease. However, when the water depth in the basin is increased without a proportional in­crease in infiltration rate, the rate of turnover of the water in the basin is decreased. In the above example, the turnover rate is once every 100/60 = 1.7 days for the 100-cm depth, but once every 400/50 = 8 days for the 400-cm depth. This means that, after increasing the water depth, the water stays longer in the basin and algae in the water are exposed longer to sun­shine. This causes more growth of algae, particularly of unicellular sus­pended algae, like Carteria klebsii, which can be very troublesome because they are physically strained out at the top of the clogging layer on the bottom as water infiltrates into the soil (Bouwer and Rice 1984). These algae then form a filter cake which further reduces infiltration rates. Also, where algal concentrations are high, considerable increases in the pH of the water in the basin occur due to uptake of dissolved carbon dioxide by the algae when they photosynthesize during daylight. This can cause pH values to climb to a range of 8 to 10, where calcium carbonate begins to precipitate out. This calcium carbonate accumulates on the bottom of the basins, where it aggra­vates the clogging problem. Thus, even though in the example compaction of the clogging layer by increasing the water depth produced only a slight decrease in infiltration rate, the subsequent "secondary" effects such as algae growth and calcium carbonate precipitation can reduce the infiltration rates much further. This is in marked contrast to the significant increase in infil­tration rate that might intuitively be expected when the water depth is in­creased that much!

For the second scenario, the restricting layer is assumed to consist of less compressible fine sand and silt with an E-value of 50 kg/cm2. Taking the thickness of the fine-sand layer again at 10 cm and again assuming a pressure head of -20 cm water at the bottom of the layer, AP, due to an increase in water depth from 100 cm to 400 cm again is 150 g/cm2 = 0.15 kg/cm2. However, the compaction z calculated with Eq. 1 is now 0.15 X 10/50 = 0.03 cm, leaving the clogging layer with a thickness of 9.97 cm. This would reduce the void ratio from 5/5 = 1 to 4.97/5 = 0.994. Figure 13 in Ter-zaghi and Peck (1948) shows that for sands, this would give a hydraulic conductivity reduction of about 2% for the clogging layer. Taking K of the clogging layer before compression as 15 cm/day, K of the clogging layer after compression would thus be 14.7 cm/day. For the initial water depth of 100 cm and K of the clogging layer before compression equal to 15 cm/ day, the infiltration rate would be 15 X (100 + 10 + 20)/10 = 195 cm/ day. After increasing the water depth 4 times to 400 cm and reduction of K of the clogging layer to 14.7 cm/day, the infiltration rate would be 14.7 X (400 + 9.97 + 20)/9.97 = 634 cm/day or 3.25 times greater than before increasing the water depth. These calculations show that for the less com­pressible clogging layers on the wetted perimeter of infiltration basins, in­creasing the water depth can significantly increase infiltration rates. The turnover rate of the water in this example decreased from once every 0.51 days to once every 0.63 days. It is doubtful that this would significantly increase algae growth.

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Clay particles are colloids, which are surrounded by the so-called double layer due to cation adsorption, and by one or two layers of water molecules due to the polarity of such molecules. The water in these layers has what has been called an ice-like structure (Low 1959) and is essentially immobile. For certain clays, this immobile water can occupy about half the pore space, leaving only the other half of the pore space available for water movement. When such a clay is compressed, the loss of pore space for mobile water is about twice the reduction in total pore space. This could produce large de­creases in hydraulic conductivity.

Clogging of soil in recharge basins is a very complex phenomenon that not only consists of accumulation of sediment on the wetted perimeter, but also of biological activity (algae, microorganisms, small or large animals), entrainment of fines into the underlying soil, formation of gases that could stay entrapped and decrease the hydraulic conductivity of the clogging ma­terial or move through the clogging layers as bubbles and increase the hy­draulic conductivity, biodegradation, chemical effects, etc.

Clogging layers do not have to be several centimeters or several deci­meters thick. They can also be very thin. For example, they may consist of a very thin deposit of fine soil material on the bottom of the recharge basin due to dusty storms, a short event of increased suspended solids in the water entering the basins, or stirring up of bottom material when the basin is filled after a drying and cleaning period. Basins must always be filled very care­fully, with minimum disturbance of bottom soil; otherwise, the filling op­eration becomes the start of the next clogging process. Algal or bacterial films with their metabolic products can also develop on the bottom (biofilms, biofouling), and these films can be very thin. Where the clogging layer is very thin, its thickness Ls and hydraulic conductivity Ks are hard to char­acterize individually. Thus, these parameters are lumped together into one parameter called the hydraulic impedance Rs and calculated as Ls/Ks. Values of Rs are determined by solving Darcy's equation for the flow across the clogging layer as

i

where i is the infiltration rate (Darcy flux) and h is the total head loss across the clogging layer (water depth minus pressure head below clogging layer). Rs has the dimension of time, for example, days, and it can be construed as the number of days required for a unit amount of water to move across a clogging layer at unit hydraulic gradient.

FIELD STUDIES

The Flushing Meadows project was an experimental project on ground­water recharge with secondary effluent. It was located in the Salt River bed west of Phoenix and consisted of six test basins 6.7 x 210 m each. It was used to evaluate optimum design and management criteria for larger projects. The sewage effluent was essentially free from algae and normally had a low suspended solids content (about 10 mg/L). The main objective of the system was to serve as a test project for advanced treatment of wastewater by per­colating it through the vadose zone and aquifer, so that the water after re-

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covery from the aquifer could be used as such for unrestricted irrigation and recreation or, with further treatment, for drinking. Thus, the operational sys­tem basically would be a recharge-recovery or soil-aquifer-treatment (SAT) system (Bouwer et al. 1974a, 1974b, 1980). Experiments were carried out for a 10-year period (1967-1977). For the first five years, the main objective was to see how hydraulic loading could be maximized. For the second five years, the main objective was to see how denitrification in the soil could be maximized to remove as much nitrogen from the wastewater as possible.

The soil in the infiltration basins was a fine loamy sand for the first meter and then coarse sand and gravel plus boulders to a depth of about 85 m where a clay layer began. Hydraulic loading rates were about 100 m/yr, using flooding periods of 9 to 14 days and drying periods of 12 to 14.days. During flooding, average infiltration rates were about 0.6 m/day. There was some decline in infiltration rates during flooding because of accumulation of mostly organic, sludge-type material on the bottom. Infiltration rates were restored by the regular drying cycles and by cleaning the basins about once a year by shaving the accumulated organics from the basin bottoms with a front-end loader.

The water depth in the basins was kept at about 20 cm. Thus, the turnover rate of the water in the basins was high (about once every eight hours), and suspended algae did not develop. However, there was growth of filamentous algae on the bottom. Oxygen produced during photosynthesis partly stayed entrapped in the algal mat, causing pieces to break loose and float up to the surface. These floating pieces of algae carried bottom sediment up with them, leaving a completely clean bottom where they had broken loose. This pro­duced a natural rejuvenation of the bottom and a restoration of infiltration rates, especially during summer. The floating algal pieces moved to the out­let of the basins where they returned to the sewage effluent stream, or they were blown by winds to one end of the basins where they could be period­ically removed during drying. The Flushing Meadows project was a case where, because of high turnover rates and low algae contents of the sewage effluent, clogging of the bottom soil was not a big problem. As a matter of fact, the filamentous algae helped restore infiltration rates during flooding and thus had a beneficial effect.

Algal problems were encountered, however, in another project on ground­water recharge with sewage effluent, installed in 1975. This was a 16-ha demonstration project with four basins of 4 ha each. It was located in the Salt River bed in Phoenix and received secondary sewage effluent from the 23d Avenue sewage treatment plant after it had passed through a 32-ha sta­bilization pond (Bouwer and Rice 1984). Since the detention time in this pond was on the order of three days, there was considerable growth of algae, especially during the summer when the outflow from the stabilization pond entering the recharge basins was pea-soup green with suspended algae, mostly Carteria klebsii. In view of anticipated clogging of the bottom soils in the infiltration basins due to the algae, the infiltration basins were constructed so that the water depth could be relatively large (about 1 m) to provide more pressure head on the bottom, presumably to overcome the flow-restricting effect of clogging algae layers on the bottom. When operation of the system began, however, hydraulic loading rates at flooding and drying periods of two weeks each were disappointingly low, i.e., only about 21 m/yr, whereas

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the Flushing Meadows project for similar soil conditions yielded hydraulic loading rates of about 100 m/yr.

Studies indicated that because of the 1 m depth in the basins and the low turnover rates (about once every 10 days), there was considerable additional growth of suspended algae in the infiltration basins, which formed a filter cake on the bottom and caused precipitation of calcium carbonate. Air pres­sure buildup in the vadose zone below the large infiltration basins was also considered as a possible cause for the low hydraulic loading rates. However, air pressure measurements in the vadose zone showed that this was insig­nificant. Even after the start of a new flooding period, air pressures in the vadose zone rose only very slightly and soon returned to atmospheric. Thus, algae growth was the main reason for the low infiltration rates in the basins.

To remedy the situation, a dual attack was used. Firstly, a bypass channel was constructed around the 32-ha stabilization pond to get the secondary effluent quickly into the infiltration basins without chance of algae growth in the water. Secondly, the water depth in the infiltration basins was reduced from 1 m to 0.2 m to increase the rate of turnover of the effluent in the infiltration basins, thus minimizing algae growth in the infiltration basins themselves. The two measures were effective in reducing the clogging prob­lem. Subsequent hydraulic loading rates were about five times higher than before, i.e., about 100 m/yr instead of 20 m/yr (Bouwer and Rice 1984).

Occasionally, operators of recharge systems have increased the water depth in their basins to increase infiltration rates. Sometimes, these efforts have been successful. In one case, for example, the bottom of the basin was below the groundwater table. The basin could never be dried and cleaned, and a thick layer of sediment (on the order of 0.5 to 1 m thick) had accumulated on the bottom. The banks were very steep and had much less sediment ac­cumulation. Thus, most of the infiltration took place through the basin banks and, as could be expected, increasing the water depth in the basins signif­icantly increased the infiltration rates. In another case, however, a number of small basins were consolidated into a few large basins and in the process, considerable bottom material was excavated so that the water depths in the new basins were considerably greater than before. The new infiltration rates, however, were considerably less than before, probably due to compression of the clogging layer on the wetted perimeter.

LABORATORY STUDIES

The effects of clogging material and increasing the water depth on infil­tration rates were also studied in the laboratory, using two soil columns. The columns consisted of 10-cm ID PVC pipe and were 340 cm long. They were filled with 240 cm of loamy sand with a 6-cm layer of pea gravel at the bottom for drainage. Tensiometers were located at distances of 5, 25, 65, 145, and 225 cm from the soil surface. The columns were saturated from below, after which a 20-cm water depth was maintained above the soil sur­face with a Mariotte siphon arrangement, using tap water. The columns were allowed to drain freely into the 6-cm gravel layer at the base.

Infiltration was continued for about two weeks to allow infiltration rates to become relatively constant. Then, a loam suspension was added to each column to form a clogging layer on the soil surface of about 1 cm thickness and a bulk density of 1.5 g/cm3. This reduced the infiltration rates from 53

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cm/day to 35 cm/day (average for both columns). The impedance of the clogging layer, calculated as head loss across clogging layer divided by in­filtration rate, was two days. One week after the addition of the loam layer, the water depth in one column was increased from 20 cm to 85 cm, while the water depth in the other column was maintained at 20 cm as a check. The increase in water depth to 85 cm increased the infiltration rate from 30 cm/day to 40 cm/day in about one day, and to 45 cm/day after one week, averaging 42 cm/day for the entire week. However, the impedance of the clogging layer on the soil was increased from 2 days to 3.8 days, due to the increase in intergranular pressure Pt and resulting compression of the clogging layer and decrease in hydraulic conductivity. Thus, the water depth increase from 20 to 85 cm did not produce the infiltration rate increase from 30 cm/day to 70 cm/day that would have occurred if the impedance of the clogged layer had remained constant at two days, but only an increase from 30 cm/day to 42 cm/day. This small increase reduces turnover rates and, for a real system, would make it vulnerable to secondary effects. The study demonstrated that clogging layers on the bottom of recharge basins can in­deed undergo significant reductions in hydraulic conductivity due to compression of the layer when the water depth is increased.

After the studies with the loam as clogging material, the loam layers were removed and the experiment was repeated using an organic, mucky material to form the clogging layer. The material was obtained from the bottom of a small recreational lake in Scottsdale, Arizona. Infiltration rates were around 60 cm/day prior to adding the muck and 30 cm/day shortly after adding the muck, which formed a layer of about 5 cm thickness. Then the infiltra­tion rates increased to about 60 cm/day in a 17-day period, using the same water depth of 20 cm. This increase in infiltration rate could be caused by biological activity in the muck layer and the formation of gasses such as methane, hydrogen sulfide, and nitrogen. The resulting gas bubbles could have loosened up the layer as they worked their way up and escaped through the water. Also, there may have been some biodegradation of clogging ma­terial. Increasing the water depth to 85 cm immediately increased the infil­tration rate to 110 cm/day, after which the infiltration rate decreased to 70 cm/day in about one day. Apparently, there was some lag in the compres­sion of the muck layer. However, as infiltration was continued at the 85-cm depth of water, the infiltration rate continued to rise slowly, reaching a value of 110 cm/day after 10 days. Again this was apparently due to bio­logical activity in the muck layer with the gas bubbles continuing to produce loosening-up effects, and/or biodegradation of clogging material. Imped­ances of the muck layer ranged between 0.7 and 2 days. This study showed that organic clogging layers in recharge basins and the resulting infiltration rates may show an entirely different response to increasing the water depth than do accumulations of inorganic fines.

CONCLUSIONS

Increasing the water depth in infiltration basins for groundwater recharge can produce a more than linear increase in infiltration rate, an essentially linear increase, a less-than-linear increase, no increase, or even a decrease, depending on the conditions controlling the infiltration process. While an analysis of such conditions could give some indication of how water depth

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in recharge basins may affect infiltration rates, effects of water depth on infiltration rates are very site specific and the optimum water depth for max­imum hydraulic loading rates is best determined with on-site experimentation using existing operational basins or special test basins. In general, shallow basins (water depth about 10 to 30 cm) well above the groundwater table may be the most desirable from a standpoint of infiltration rate and ease of maintenance (rapid drying) for maximum hydraulic loading. Also, in a mul­tiple-basin system, basins should be hydraulically independent so that they can be flooded, dried, and cleaned according to their best schedules.

ACKNOWLEDGMENTS

This paper is a contribution of the Agricultural Research Service, U.S. Department of Agriculture.

APPENDIX. REFERENCES

Bouwer, H., Rice, R. C , and Escarcega, E. D. (1974a). "High-rate land treatment: I. Infiltration and hydraulic aspects of the Flushing Meadows project." J. Water Poll. Contr. Fed. 46(5), 835-843.

Bouwer, H., Lance, J. C , and Riggs, M. S. (1974b). "High-rate land treatment. II. Water quality and economic aspects of the Flushing Meadows project." J. Water Poll. Contr. Fed. 46(5), 844-859.

Bouwer, H. (1978). Groundwater hydrology. McGraw-Hill Book Company, New York, N.Y.

Bouwer, H., et al. (1980). "Rapid-infiltration research—the Flushing Meadows proj­ect." J. Water Poll. Contr. Fed. 52(10), 2457-2470.

Bouwer, H. (1982). "Design considerations for earth linings for seepage control." Ground Water 20(5), 531-537.

Bouwer, H., and Rice, R. C. (1984). "Renovation of wastewater at the 23rd Avenue Rapid-Infiltration Project, Phoenix, AZ." / . Water Poll. Contr. Fed. 56(1), 76-83.

Low, P. F. (1959). "Viscosity of water in clay systems." Clay Mineralogy 8, 170-182.

Terzaghi, K., and Peck, R. B. (1948). Soil mechanics in engineering practice. lohn Wiley & Sons, New York, N.Y.

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