Urban land use impact on soil heavy metal levels in Lafayette, Louisiana (USA)

TL;DR

This study examines how different land uses in Lafayette, Louisiana, affect soil heavy metal levels, finding higher concentrations in industrial and roadside areas.

Contribution

The study provides a detailed characterization of heavy metal contamination in a mid-sized U.S. city, emphasizing land-use impacts and localized hotspots.

Findings

Industrial and roadside areas showed the highest heavy metal enrichment, particularly for Pb and Zn.

Only 7% of samples exceeded the EPA's Pb screening level, with low overall ecological risk despite localized hotspots.

Soil properties like pH and SOM had weak correlations with metal concentrations at the city scale.

Abstract

Urban soils can concentrate metal(loid)s from legacy and ongoing urban activities, yet mid-sized U.S. cities remain under characterized. We quantified As, Cr, Cu, Mn, Ni, Pb, and Zn in 1,290 composite topsoils (0–2.5 cm) across five land uses in Lafayette, Louisiana (USA), and measured pH, Electrical Conductivity (EC), and soil organic matter (SOM). We compared concentrations to local geochemical backgrounds and screening levels and calculated Pollution Index (PI), Pollution Load Index (PLI), Geoaccumulation Index (Igeo), Single Ecological Risk Index (Ei), and Potential Ecological Risk Index (PERI). Metals were highly heterogeneous (e.g., max: Pb = 6,877; Zn = 6,776; Cr = 3,024; Mn = 20,826 mg/kg). Relative to background, exceedance rates were: Zn 99%, Cr 93%, Mn 92%, Pb 83%, Cu 60%, As 54%, Ni 15%. Only 7% of samples exceeded the current USEPA Pb residential soil screening level (200 mg/kg); none exceeded Cu, Ni, Zn, or Mn screening values. Land-use patterns were clear: Industrial > Roads > Residential > Parks/Gardens, with Pb and Zn driving most enrichment. Spearman correlations showed a Pb–Zn–Cu cluster (r up to 0.58) and weak associations between metals and pH, EC, or SOM (|r| ≲ 0.18), indicating limited control of basic soil properties at city scale. PCA and clustering similarly grouped Pb–Zn–Cu, while Ni remained near background. Mean PI values were highest for Pb (6.27) and Zn (6.24); mean PLI = 2.14 (anthropogenic signal), whereas PERI indicated low overall ecological risk (mean = 10.85) with localized hotspots. Results show that Lafayette exhibits a low-to-moderate, legacy-dominated contamination profile, principally Pb and Zn in industrial and roadside settings predominantly in the city center, while parks and gardens are comparatively clean. Findings support targeted risk communication and site-specific management rather than city-wide remediation.

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