A field-scale remediation of residual light non-aqueous phase liquid (LNAPL): chemical enhancers for pump and treat

Abstract

The remediation of petroleum-contaminated soil and groundwater is a challenging task. The petroleum hydrocarbons have a long persistence in both the vadose zone and in the aquifer and potentially represent secondary and residual sources of contamination. This is particularly evident in the presence of residual free-phase. Pump-and-treat is the most common hydrocarbon decontamination strategy. Besides, it acts primarily on the water dissolved phase and reduces concentrations of contaminants to an asymptotic trend. This study presents a case of enhanced light non-aqueous phase liquid (LNAPL) remediation monitored using noninvasive techniques. A pilot-scale field experiment was conducted through the injection of reagents into the subsoil to stimulate the desorption and the oxidation of residual hydrocarbons. Geophysical and groundwater monitoring during pilot testing controlled the effectiveness of the intervention, both in terms of product diffusion capacity and in terms of effective reduction of pollutant concentrations. In particular, non-invasive monitoring of the reagent migration and its capability to reach the target areas is a major add-on to the remediation technique. Most of the organic contaminants were decomposed, mobilized, and subsequently removed using physical recovery techniques. A considerable mass of contaminant was recovered resulting in the reduction of concentrations in the intervention areas.

Keywords: Contaminant remediation; Hydrocarbon contamination; Hydrogeophysical monitoring; Light non-aqueous phase liquid desorption; Pilot test; Residual hydrocarbons.

Fig. 1

Configuration of pilot test and location of injection piezometers (PZI, PZ15, PZ11)

Fig. 2

Three-dimensional geological model (with vertical exaggeration) of the Decimomannu military airbase and location of injection points (PZI, PZ11, PZ15). The dashed line identifies the area used for the storage of fuel tanks

Fig. 3

Concentration of total petroleum hydrocarbons detected in the piezometers located inside the area used for the storage of the fuel tanks over time

Fig. 4

ERT time-lapse results during different field test activities representing the LTP1 line, which covers the PZ11 injection point. The injection point corresponds to the vertical blue arrow

Fig. 5

ERT time-lapse results at different stages of reagent application corresponding to the LTP3 line, which covers the PZI injection point. The blue arrow indicates the injection point (a), the ascent of the product along the piezometric tube (b)

Fig. 6

Analysis of water samples recovered during the phases of the pilot test monitoring over time

Fig. 7

Mass spectrometry and gas chromatography characterization of water samples recovered during the implementation of the experiment at the field scale over time