Large Scale Industrial Fluid Waste Injection

Goolsby (1971) described a thorough monitoring project of a large-scale industrial waste injection project in Florida. The task was to inject fluid waste at a rate of up to 500m3/h and about 3 x 106m3/year. The fluid composition was described as an aqueous solution of organic acids, nitric acid aminos, alcohols, ketones, and inorganic salts.

Discussion. The injection site was selected because of geological and hydrological considerations (Fig. 16.6):

• A free surface sand and gravel aquifer was found to overlay two major confined aquifers of Floridian limestone, the three being separated by distinct clay layers.

• The water in the upper limestone aquifer contained 425 mg Cl/l, whereas the lower limestone aquifer contained 7900 mg Cl/l. Thus the two aquifers did not communicate hydraulically.

• The water of the lower limestone aquifer was nonpotable.

• In no place was the saline lower aquifer water seen to ascend to the surface.

Fig. 16.6 Generalized geological cross-section through southern Alabama and north-western Florida. An injection well penetrated the confined lower limestone aquifer. (After Goolsby, 1971.)

• The recharge area of the lower aquifer was about 120 km to the north.

• The nearest possible discharge area of the lower aquifer, the ocean, was about 160 km south of the selected injection area. Thus the injection site was remote from recharge and discharge points, and the injected aquifer was saline and isolated from the overlying aquifer.

Injection and monitoring systems. An injection well (A in Fig. 16.7), 500 m deep, was drilled into the lower confined limestone aquifer, and later a second well (B) was drilled 400 m west of the first one. A deep monitoring well, 400 m south of A, was drilled into the receiving aquifer. A shallow monitoring well was drilled into the upper limestone aquifer, 30 m from injection well A. Hence, changes in the chemical composition could be monitored both in the receiving and overlying aquifers.

Water levels recorded in the two injection and two observation wells are given in Fig. 16.7, along with the injection rates. The following observations were made:

• The deep monitoring well responded with an increase in water level as a function of the rate (and pressure) of injection.

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Fig. 16.7 Piezometric water levels in waste injection and monitoring wells, and injection rates in Florida. (From Goolsby, 1971.)

Fig. 16.7 Piezometric water levels in waste injection and monitoring wells, and injection rates in Florida. (From Goolsby, 1971.)

• The shallow monitoring well revealed no change in water level, indicating the upper limestone aquifer was not affected.

The calcium concentration in the deep monitoring well is given in Fig. 16.8. The following patterns were observed:

• Although the waste fluid was low in calcium (about 3 ppm), it was calcium tagged in the aquifer. This was actually expected because of the reaction of the acid waste (pH 5, and later pH 3) with the aquifer limestone.

• The first front of injected waste arrived 10 months after commencement of the operation. The arrival of the fluid in the 400-m-distant monitoring well was noticed by a marked increase in calcium (Fig. 16.8).

• At the beginning of 1966 the calcium concentration decreased in the deep monitoring well, a feature not well understood.

• In 1968 a previous neutralizing process was stopped, and the waste was injected with a pH of 3, resulting in a sharp increase in dissolved calcium in the monitoring well (Fig. 16.8). This indicated that limestone was dissolved at a high rate. The resulting "karstification" probably enhanced the movement of the waste in the aquifer.

Additional chemical data are given in Table 16.1. As mentioned, the composition of injected waste was changed in the middle of 1968, when neutralization was dropped. As a result, the pH went down to 3.3 and the nitrate concentration went up. The deep monitoring well responded in the following way (Table 16.1): the October 1967 and May 1968 analyses

Fig. 16.8 Calcium concentration, deep monitoring well, 1963-1964, Waste Injection Projection, Florida. (From Goolsby, 1971.)
Table 16.1 Composition (mg/l) of Waste and of Groundwater

Injected wastes

Deep monitor well

Nov 1967

Apr-Dec 1968 (average)

Oct 1967

May 1968

Jan 1969

Ca

3.4

<20

168

165

2350

no3

2420

5070

2

0

5760

Na

610

<1000

3720

3720

635

Cl

82

200

5700

5900

161

COD

22980

2355

22100

pH

5.2

5.3

6.80

7.00

4.75

Source: From Goolsby (1971).

Source: From Goolsby (1971).

showed high sodium and chlorine concentrations, revealing dominance of the original saline aquifer water.

The percent wastewater was calculated by the investigators from the chlorine concentration in the monitoring well (5800 mg/l) and the initial chlorine concentration in the lower limestone aquifer (7900 mg/l):

. . J 7900 - 5800 1AA „,n/ percentage injected water :-j^qq-x 100 = 26%

In other words, up to May 1968 the chlorine-rich aquifer water was diluted by 26% with injected chlorine-devoid water. The newer, nonneutralized waste with pH 3.3 caused a remarkable change in the composition of the fluid at the deep monitoring well: chlorine dropped to 160 mg/l, indicating the deep monitoring well was dominated by 98% injected water:

7900

The rapid increase from 26 to 98% waste fluid in the deep monitoring well indicated that the nonneutralized waste dissolved the aquifer carbonates and developed a highly conductive karstic system, which was very alarming in terms of waste disposal in the aquifer. During all this time no chemicals arrived in the upper monitoring well.

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