Investigation of Cu and Zn into Farmlands Irrigated with Livestock Wastewaters Using Nemerow Pollution Index

Research Article

Austin J Environ Toxico. 2024; 10(1): 1046.

Investigation of Cu and Zn into Farmlands Irrigated with Livestock Wastewaters Using Nemerow Pollution Index

Shui-Wen Chang Chien¹; You-Cheng Chen¹; Cheng-Chung Liu²*

¹Department of Environmental Engineering and Management, Chaoyang University of Technology, Taichung, Taiwan

²Department of Environmental Engineering, National Ilan University, Ilan, Taiwan

*Corresponding author: Cheng-Chung Liu Department of Environmental Engineering, National Ilan University, Ilan, Taiwan. Tel: (0118863) 935-7400 (O); Fax: (0118863) 936-7642 Email: ccliu@niu.edu.tw

Received: November 29, 2023 Accepted: January 03, 2024 Published: January 10, 2024

Abstract

Animal husbandry flourishes in Yunlin County, Taiwan. Livestock wastewater has been commonly used as a fertilizer for decades. Such wastewater usually contains large amounts of Cu and Zn; however, over-irrigation deteriorates farmland and pollutes groundwater. Lunbei (ranches), Shuilin (pig farms), and Huwei (pig farms) townships were selected in the county for this study. To determine the accumulation of Cu and Zn in soils after irrigation, the single-factor pollution index (PI) and Nemerow pollution index (PN) were used to evaluate the pollution level and metal hazard at several monitoring sites within the irrigation group over four years. Furthermore, the data were used to identify sites with a high pollution potential. Speciation investigations of Cu and Zn were conducted to analyze their potential environmental threats. All PI levels were below 1.0, indicating that all the monitoring points were non-polluting. Although the calculated PN levels increased annually, the PN levels obtained in the third year were all lower than the safety threshold of 0.7. After irrigation, most of the available Cu and Zn in the soil were bound to organic matter and Fe-Mn oxyhydroxide, possibly posing environmental risks. Cu had a higher threat potential than Zn. The potential threat caused by irrigation with cow manure urine was more than that caused by irrigation with pig manure urine.

Keywords: Copper; Livestock wastewater; Nemerow index; Zinc

Capsule

The Nemerow pollution index is useful for monitoring the accumulation of toxic metals in soil.

Introduction

Animal husbandry is an important agricultural activity in Taiwan, accounting for 32% of the entire agricultural production as reported in the Annual Report of the Council of Agriculture, Executive Yuan (Taiwan). However, considerable amounts of wastewater are produced daily in animal husbandry. In earlier years, they were discharged into rivers or ditches without treatment, causing severe environmental pollution [1]. Livestock wastewater is rich in nutrients and organic matter, including nitrogen, phosphorus, and potassium. In addition, wastewater utilized as a fertilizer increases soil fertility and therefore lowers the use of chemical fertilizers, as well as achieves waste resource recovery.

In recent years, authorities in Taiwan’s agricultural affairs have implemented a livestock wastewater reuse policy. In 2009, the Council of Agriculture cooperated with researchers to conduct a test program for livestock wastewater reuse to encourage resource recovery from livestock wastewater [2]. Livestock and cropland farmers were invited to participate in forums and observation tours to understand the policy. The Environmental Protection Administration has also subsidized local governments to buy vehicles, irrigation tools, and storage tanks to encourage farmers to irrigate their croplands with livestock wastewater. However, Cu and Zn are commonly added to livestock feed as growth regulators to reduce growth time and prevent diseases [3]. Only 10–20% of Cu or Zn can be absorbed into the livestock body, but the residual Cu and Zn are excreted through manure and finally transferred to the livestock wastewater [4,5]. When considerable amounts of Cu and Zn are continuously brought into farmland through irrigation, accumulation occurs, encouraging their over-uptake. Eventually, metals are transferred to the human body through the food chain, posing health risks [6,7]. Heavy metals are considered persistent environmental pollutants. Organisms are difficult to degrade and thus have longer biological half-lives. A test result of Broadbalk agriculture from Rothamsted Research showed that the Cu and Zn concentrations in soil increased by approximately 60% after composting over 160 years [8]. Therefore, the long-term monitoring of irrigated croplands is necessary.

The Nemerow pollution index (PN), which helps determine heavy metals posing the greatest risk to a single environment, has been used to assess the effect of heavy metals on contaminated rivers or the effect of groundwater on plant growth in paddy soils [9]. An area with a high PN level indicates the area is severely polluted and requires increased attention [10,11]. Risk assessment of cropland to heavy metals allows us to determine the degree of pollution, which is classified as a high-, middle-, or low-risk indicator. This is also used to evaluate the extent to which cropland is used for agriculture if Cu and Zn consistently accumulate in the soil [12]. The single-factor (PI) and the Nemerow (PN) pollution indices are typical environmental indicators [13]. PI calculates the pollution level of each metal in the soil, and PN the comprehensive pollution impact of heavy metals [14]. They have been used to evaluate the potential of heavy metals in farms in Taiwan [15], to calculate the distribution of metalliferous sediments on the surface of the Yellow River in China [16], and to estimate the pollution of soil near steel mills in Iran [17].

This study aimed to use the Nemerow pollution index to (1) examine the accumulation of Cu and Zn in croplands after irrigation using livestock wastewater, (2) obtain evaluation data by calculating the Cu and Zn concentrations through indicators, (3) grade the soil using the monitoring point, and (4) identify the form of Cu and Zn in the irrigated croplands. These data were summarized, and the potential environmental threats through long-term irrigation were evaluated.

Methodology

Survey and Sampling Areas

Yunlin County is among the top three animal producers in Taiwan. Based on a report from the Council of Agriculture in 2020, the Yunlin area produces 28.4% of pigs in Taiwan. Therefore, Yunlin was selected as the survey area in this study. Three townships in this county, namely, Lunbei (ranches), Shuilin (pig farms), and Huwei (pig farms) were identified as major sampling areas (Figure 1). The document defines “irrigation” as using livestock wastewater as fertilizer in specific croplands. Irrigation in the Shuilin and Huwei areas involved pouring wastewater from a tank car and flooding the cropland, whereas in the Lunbei area, a tank car was coupled with furrow irrigation. Sampling was conducted during the dry season (March and May) and the rainy season (July and September) because the elemental contents of soil are easily affected by moisture. Table 1 shows the farm type, type of irrigation wastewater, number of monitoring points, and monitoring period for the experimental areas. The term “spare monitoring point” was defined as an irrigated monitoring point without livestock wastewater. Solid–liquid-separated raw water containing solid manure was used to irrigate the Lunbei area. Anaerobic swine wastewater that underwent a two-stage treatment without solid manure was applied to the Shuilin area. Aerobic swine wastewater that underwent a three-stage treatment and contained solid manure was used to irrigate the Huwei area.