Contamination by Released Oil Brines

Fryberger (1975) described a case study of groundwater contamination by displaced oil production brines. In 1967 a farmer lost his rice crop near Garland City, southwest Arkansas, USA, because his well water became saline. An adjacent oil field came into production in 1955, with an increasing amount of separated brine that had been diverted into a poorly lined evaporation pond that leaked into the local aquifer. The brine was subsequently recharged into the ground through an abandoned production well, but soon the casing became corroded and the brine again reached the local aquifer. Eventually a new recharge well was constructed.

Monitoring project. To monitor the brine contamination, 36 observation wells were drilled to different depths. Chlorine, regarded as best representing the brine, was measured, and the data served to produce a chlorine concentration contour map (Fig. 16.10). The spread of the lines indicates the water in the aquifer flowed southward. A section showing chlorine distribution (Fig. 16.11) also indicated a southward flow. To get a more complete picture of the brine movement, a schematic compositional cross-section was prepared (Fig. 16.12). The results were plotted as a function of the distance of each observation well from the contamination center. The concentration of Cl, Br, F, Pb, Ca, Mn, Ni, and Al decreased in a pattern explainable by mixing, in various proportions, of contaminating brine and local groundwater. However, several dissolved ions revealed different patterns, indicating secondary processes occurred:

• Iodine, iron, and zinc increased in wells over the concentrations in the brine or local groundwater, represented by the private (PVT) well. The researchers hypothesized that chemical displacement of ions coating the sand grains took place. This explanation is plausible for the iron alone. Hence, another explanation was

Fig. 16.10 Isochloride contours at the bottom of the alluvium aquifer, Gerald City. Flow to the south is seen. (From Fryberger, 1975.)
Fig. 16.11 A chloride concentration cross-section, Gerald City. Leakage from the brine evaporation pit is recognizable. The heavy brine accumulated above the shale aquiclude. (From Fryberger, 1975.)
Tracing Alphabet
Fig. 16.12 Composition histograms (mg/l) of the brine pit of Gerald City and monitoring wells, arranged by increasing distance from the pit. A contamination gradient is observed from the brine pit toward the fresh water well PVT. (From Fryberger, 1975.)

offered—that the brine disposed in earlier stages had a different composition.

• Barium is seen in Fig. 16.12 to drop faster than chlorine in wells TW 11 and TW 20, indicating effective precipitation en route.

• SO4 is high in the private well and TW 20, indicating it is high in the local groundwater. The observed drop in SO4 concentration in TW 11 and TW 3 is probably due to consumption by barium precipitation.

Discussion. The term conservative is applied to dissolved ions that, except for mixing, take part in no other process. In contrast, nonconservative ions reveal changes in concentrations caused by mixing of two water types and, in addition, by water-rock interactions. Looking at Fig. 16.12, which ions are conservative and which are nonconservative? Cl, Br, Ca, Ni behaved as conservative ions. In contrast, secondary reactions changed the composition of the water by exchange reaction with rocks, for example, with Fe and possibly Zn.

Conclusion. Different ions travel at different velocities, the conservative ions moving fastest. The conservative ions are most helpful for the understanding of groundwater flow.

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