The Geochemical Characteristics Of The Soils And Red Clay Layers In The Yellow River Delta And Their Environmental Implications

The Yellow River Delta（YRD） is heavily influenced by natural processes（e.g., Yellow River flow-sediment variation, river course transition and global climate change） and human activities（e.g., cultivation, urbanization and industrialization）. A systematic knowledge is still limited on the soil genesis and development process, soil environmental quality, and the material relationship between soil and sediment under the conditions of climatic change, human activities and land-ocean interaction. Forty-two representative soil profiles with 182 soil layer samples and 26 surface（0-20cm） sediment samples were collected from the YRD and its adjacent offshore areas. All these samples were measured for soil properties, trace elements and rare earth elements, organochlorine pesticides, total petroleum hydrocarbon, radioactive lead isotope and stable carbon and nitrogen isotopes. Soil colloids were extracted from some bulk soils and were analyzed for the geochemical characteristics. The measured results have been employed to characterize the soil environment quality from inland to the coastal area, to investigate the environmental geochemical behavior and possible sources of the inorganic elements and organic pollutants, to reveal the spatial and temporal distribution, geochemical characteristics and environmental implications of the red clay layer（RCL） occurring in the soil profiles of the YRD. These results are expected to be useful in understanding the biogeochemical cycles and sustainable development of the YRD under high pressure of human activities and strong land-ocean interactions. The main results were summarized as follows:（1） Change of the land-use types from coastal beach to inland area resulted to a change of the soil properties. The soil salinity, texture and fertility status generally showed an increasing trend in the order of tidal flat, wetland, cotton field, cereal field, and vegetable field. The lowest soil fertility in the tidal flat indicated the primitive characteristics of natural soil physicochemical properties. After the tidal flat developed into wetland, the contents of clay, organic matter, nitrogen and phosphorus were all significantly enhanced due to the flooded conditions and increased vegetation cover. Some of the tidal flat were reclaimed into farmland and the cultivation process facilitated soil desalination and dealkalization, and improved the input of nitrogen and phosphorus into the soils. However, the cultivation meanwhile destroyed soil aggregates and promoted the mineralization of soil organic matter.（2） The soils of the YRD were less contaminated by heavy metals, organochlorine pesticides and total petroleum hydrocarbon based on current measurements. The mean concentrations of the heavy metals were elevated along the Yellow River and in the southern part of the delta. The percentages of γ-HCH and o,p’-DDT as less recalcitrant components tended to distribute in the coastal area. However, the more recalcitrant components, β-HCH and p,p’-DDE, tended to distribute in the inland area. The contents of the total petroleum hydrocarbon in the soils exhibited high levels in the inside of the modern YRD（especially near the Gudong oilfield）, and low levels in the outside of the modern YRD. The heavy metals accumulation in the soil profiles was related with the contents of iron oxide and clays. The clays were also important carriers of γ-HCH in the surface and subsurface soils. The distribution of total petroleum hydrocarbon was mainly related to the degree of soil development. Suspended sediments transported by the Yellow River were suggested to be one of the major sources for the pollutants accumulation in the YRD soils. However, the increased intensity of regional human activities, such as water and sediment regulation, oil exploitation, agricultural reclamation and coastal engineering, has also increasingly threatened the soil environment of the area.（3） The content of rare earth element（REE） in the soil profiles of the YRD was positively correlated with the degree of soil development. The chondrite-normalized REE distribution patterns of the different soil layers were similar in shape. All the patterns were characterized by steep La-Eu and fairly flat La-Eu profiles and the higher accumulation of the light REE compared with the heavy REE. The significant europium（Eu） depletion and slight cerium（Ce） enrichment occurred in all the soils. The patterns of soil REE implied that the original materials of the different soil layers must have undergone thorough sedimentary mixing processes during erosion, transportation and deposition.（4） The typical RCL occurred in the soil profiles of the YRD was mainly distributed in the modern YRD that formed after 1885 and in the old YRD that formed before 1855. The area between the Yellow River and Mi River was a transition zone for the RCL distribution, which was not observed in the east of the Mi River. The shallower occurring depth of the RCL generally showed a thicker deposit. According to the 137 Cs and 210 Pb dating records, the depositional time of the RCL was in the range of 1910s-1960 s. The RCL was more likely to occur in the section with high variability of deposition rate due to the hydrodynamic change of the Yellow River. The morphology features of the typical RCL were characterized by grain size=6.69±0.79φ, median grain size=11.6±7.3 μm and redness（a*） value=7.5±0.4.（5） The contents of clay minerals and calcite in the RCL were significantly higher than that in its Upper and lower yellow silt layer（YSL）, while the primary minerals contents in the RCL were significantly lower. As for the major mineral elements, the typical RCL had Si O₂=55.6±3.7%, Al2O3=13.5±1.1%, Ca O=8.18±1.03%, Fe2O3=5.49±0.83%, Mg O=2.83±0.34%, K2O=2.54±0.83% and Na2O=1.34±0.32%. As for the 19 trace elements, only Zr and Hf contents in the RCL were lower than that in the YSL, while other trace elements were all shown enrichment in the RCL. The RCL displayed higher weathering intensity with silica to alumina ratio=6.71±1.06. For the magnetic properties, the typical RCL had χfd%=8.3±1.7%, χarm=362.7±90.0×10-8 m3/kg, χarm/SIRM=67.1±15.1×10-5 m/A, SIRM/χlf=9.6±1.5×103 A/m and χARM/χlf=6.3±1.0, which indicated that the magnetic enhancement of the RCL was mainly due to its higher contents of fine pedogenic SP/SD particles. The chromaticity and magnetic properties in the soil colloids（<2 μm fraction） showed less difference between the RCL and YSL, while the compositions of clay minerals and elements were slightly different. The geochemical results suggested that formation of the RCL source materials was associated with a warmer and wetter climate in the source areas and deposition of the RCL in the YRD was combination of transportation, mixture and hydrodynamic sorting by the Yellow River. The provenance was similar between the RCL and the YSL. However, the sedimentary epeirogenic process under different stages, marine alteration and pedogensis after deposition could also lead to the geochemical differences in different soil layers.（6） The mean contents of the heavy metals in the RCL were about 1.5 times higher than that in the YSL. The Pb, Co and Cd exhibited higher proportions in reducible and exchangeable fraction respectively in the RCL than that in the YSL, indicating a higher mobility of heavy metals in the RCL. The RCL also acted as an important pool for carbon and nitrogen accumulation in subsoil. The RCL accumulated significant amount of inorganic carbon by forming carbonate under the pedogenic condition. The depth and thickness of the RCL-YSL sequence together with the course change provided information in relation to climate change and human activities such as soil and water conservation and dam constructions during the formation of the modern YRD. The stable carbon and nitrogen isotopes fractionation indicated that wetland soils, vegetable soils, cropland soils, river sediments, bay and deep-sea sediments were the six sub-systems controlling the cycling of the organic carbon and nitrogen in the YRD. A transition from natural soils to marine sediments corresponded to the flux of organic matter from a labile pool in the source regions to a more recalcitrant pool in the sink regions.