**3. Case study**

#### **3.1. Introduction**

#### *3.1.1. Semi-arid soils and tannery sludge*

Applications of tannery sludge are restricted due to high Cr content, even when Cr(III) content in the sludge is considered to be unavailable. Nevertheless, the use of tannery sludge for reforesting the north of Guanajuato (a natural reserve in Dolores Hidalgo, Guanajuato) is appealing because, it can avoid soil degradation due to the increasing erosion in the region, and at the same time, it can reduce contamination by tannery sludge dumping to open air in Leon, Guanajuato.

The influence of Cr on nitrogen transformation in soils has been studied and reports show somewhat mixed results. James and Bartlett [53] found no inhibition in treatments containing Cr(III) in sewage sludge or tannery effluent but that nitrification was inhibited by Cr(VI) at a concentration of 10 μg g−1 in soil suspensions. Chang and Broadbent [54] observed that nitrogen immobilization, mineralization, and nitrification were inhibited to a great extent by Cr(III) added to a neutral soil, but Cr(VI) was not measured in the extracting solutions used to characterize soil Cr in this study.

Previous studies in semi-arid soils collected in the same natural ecosystem with mesquite amended with tannery sludge to evaluate the biological functioning of soil, show that C and N mineralization increased. Similarly, there was not inhibition in the biological functioning of soil [2].

The aims of this research project were 1) to evaluate the environmental impact of heavy metals from tannery sludge by determining not only the total content in a matrix, but also their bioavailability and their capacity for mobilization and toxicity by chemical fractionation. 2) To indicate the effect of tannery waste type on soil aggregate stability, infiltration, runoff, sediments, nutrients, and Cr loss from semi-arid soils using a simulated rainfall system. This information will be useful for evaluating the ecological risk associated with the use of tannery sludge in semi-arid soils during reforestation projects.

Reducing exposure to contaminants from mining activities is important, especially in the old mining towns like Vetagrande, where large areas have been affected by the presence of mine tailings.

Mine tailings have chemical and physical properties limiting plant growth, like lack of organic matter or macronutrients. Usually, they are acidic, severely toxic, do not have soil structure, have low water retention and slow rates of water infiltration [31]. Organic supplements, such as compost, farmyard manure or biosolids, may be added to overcome these limitations. Moreover, the use of organic compost mixed with tailings could alter several qualities of soil such as potential mineralization of C and N from added organic waste and the mobility and availability of heavy metals in the soil.

The aims of the use of organic amendments on mine tailings were 1) to determine the mobility factor of heavy metals; 2) to evaluate the effect of different organic wastes amended in mine tailings on N and C mineralization potential; 3) to evaluate the availability of Pb and Zn in *Brasica juncea* as indicators of heavy metal availability for pollutants and two shrubs grown in mine tailings and mixed with compost
