## 4 Results
### Comparison of MICP Treatment Approaches  
In this section, the LCSA results for each MICP treatment approach are further examined for the case when \(\Delta\)V\({}_{\text{s4}}\) = 150 m/s, indicative of improvement at a distance of 3.37 m from the injection well. Results are presented in Table 1 and Figure 6 and consider two scenarios, namely (1) the performance of MICP followed by no NH4\({}^{+}\) rinsing and (2) the performance of MICP followed the use of the optimal rinsing technique identified earlier, namely the injection of 1.8 PV of rinse solutions. When computing the results with the optimal rinsing strategy, it was assumed that the mass of in situ MICP byproducts (Figure 3b) would be independent of the number of cementation treatments applied and that removal depends only on the injected rinse solution volume [51]. This assumption is reasonable because the sum of aqueous NH4\({}^{+}\) and unhydrolyzed urea remaining in the soil pore space after cementation depends only on the concentration of applied urea, which was held constant at 250 mM for all cementation injections. Though the mass of sorbed NH4\({}^{+}\) was based upon measurements obtained after nine cementation injections, given the high aqueous NH4\({}^{+}\) concentrations present after all treatments (\(\approx\)500 mM) and the relatively small cation exchange capacity (CEC) of concrete sand (CEC = 2.58 meq/100 g of soil), it is expected that these sorbed NH4\({}^{+}\) values are reasonable estimates for all previous cementation injections. Using the LCSA results in Table 1 and Figure 6, the MICP treatment approaches can be compared on the basis of environmental impact and cost. The potential impacts associated with the low-ureolytic-rate stimulation approach (Column 2) are less than those of the high-ureolytic-rate stimulation approach (Column 1) by an average of 16% across all environmental impact and cost indicators, excluding human toxicity and ozone depletion potentials. The human toxicity (i.e., noncancerous and cancerous) and ozone depletion potentials are very small and may be considered negligible for all columns. Compared to the augmented approach (Column 3), the potential impacts of the low-ureolytic-rate stimulation approach (Column 2) are lower by about 20% on average. The potential impacts of the high-rate augmented approach (Column 3) exceed those of the high-ureolytic-rate stimulation approach (Column 1) for all impact categories by about 5% on average, except water consumption, acidification potential, and particulate potential. Without rinsing, Columns 1, 2, and 3 cost approximately USD 1760, USD 1470, and USD 1800, respectively. These costs (per 0.14 m\({}^{3}\) of treated soil) reflect the use of laboratory-grade chemicals and do not consider the economies of scale that would be achieved at a field scale when using larger volumes of industrial-grade chemicals. Hence, these monetary costs are expected to be substantially refined following data obtained from future field trials, while the environmental impact observations will likely change only modestly. The social cost for each column is approximately 0.14% of the calculated life cycle cost and ranges from USD 1.87 to USD 2.71. Figure 6: Comparative LCSA results for all columns, considering the optimal NH4\({}^{+}\) rinsing scenario, wherein 1.8 PV of rinse solution is injected (which is denoted by an \"), as well as without any NH4\({}^{+}\) rinsing. The impacts and benefits associated with using the optimal rinsing technique were also similar for all columns. Eutrophication potential was greatly reduced from 50% to 67% after injecting 1.8 PV (90 L) of rinse solution. Rinsing also decreased global warming potential, smog formation potential, and social cost by an average of 2%, 3%, and 4%, respectively. All other environmental indicators (excluding human toxicity and ozone depletion potentials) were increased by 6% on average. Treatment costs increased by only USD 12 (<1%) due to the additional materials and energy required for rinsing.