Characteristics of groundwater system and assessment of groundwater vulnerability of the Tengchong volcano group in western Yunnan

WANG Yangyang; HUANG Shengdong; PAN Dong; HUANG Guiren; WANG Yu; CHANG He; PU Yue

doi:10.11932/karst20240610

Abstract：The Tengchong volcanic group encompasses a vast area and exhibits a diverse range of eruption types.It is one of the youngest volcanic areas in China.Studying the characteristics of its groundwater system is the basis for analysis and evaluation of pollution mechanisms.Ma'anshan exhibits typical genetic and morphological characteristics in the Tengchong volcanic group.The volcanic cones and craters of Ma'anshan are relatively complete.Because the lava flows exhibit no weathered layers,Ma'anshan retains the geomorphological characteristics of the volcanos with the most recent eruption.It is dated at the Late Pleistocene or Holocene.The boundary of the groundwater system is relatively clear.Starting with the geological structure and hydrogeological characteristics of Ma'anshan area,this paper summarized the boundary characteristics of groundwater system,aquifer system characteristics,aquifer flow characteristics,and characteristics of groundwater recharge and drainage in the study area.The single factor analysis was conducted to evaluate 39 groups of water samples collected.Combined with on-site investigations,pollution sources,pollution mechanisms were explored and the groundwater vulnerability of the study area was evaluated through DRASTIC model. The results show as follows.(1)The Ma'anshan area is located in the groundwater system of Bapaidaquan,bounded by the Shitoushan underground watershed at the northeast,by the groundwater barrier on the right bank of the Nandi river at the southeast,and by the recharge boundary of the Minglang river valley at the west.The Ma'anshan area constitutes an inter-river landmass composed of the Minglang river,the Nandi river,and north-east oriented catchment valley,and mainly receives infiltration and recharge from granite fissure water,weathered fissure water and fissure water from fractured structural zones.(2)Under the strong tectonic movement and volcanic eruption,the groundwater system exhibits a complex structure and changeable hydrodynamic properties.The water-collecting chambers formed by pumice aggregates,breccia,and volcanic bomb fragments in the volcanic lava accumulation,and the lava fissures connected between them,are evenly distributed in space.Each water collecting chamber is also hydraulically connected under the communication of fractures or pores.Showing obvious characteristics with layered structures,the aquifers of the mountains in the study area are all exposed in the alluvial and pluvial layers underlying the volcanic accumulations,which supports the multi-layer interactive structural characteristics of water from lava pores and from alluvial and pluvial layers.(3)Groundwater in the study area forms pipe-type runoff along faults and continuous lava channels,and forms strands of runoff in the loose accumulations along lava fissures,volcanic bombs and volcanic breccia of varying sizes and in macropores of alluvial and pluvial gravel layers.Then,runoff converges in the lava fissure zone on the gentle slope of the mountain on the east side of the Minglang river valley and in the ancient river channel at the valley edge,and is finally discharged into the Bapai spring.(4)Areas where the water quality exceeds permitted levels are mainly distributed in industrial parks,solid waste landfills and agricultural living areas.In industrial parks,the water quality exceeds permitted levels mainly due to excessive heavy metal elements.In solid waste landfills and agricultural living areas,water contains excessive pH values.According to the characteristics of pollution sources,structure and water-containing media,aquifer anti-fouling performance,and conversion process between surface water and groundwater,there are two main types of groundwater pollution mechanisms in the study area:trans-flow infiltration and direct infiltration.(5)The proportions of low-vulnerability areas,medium-vulnerability areas,and high-vulnerability areas in the study area are 3.6％,42.1％,and 54.3％respectively.The high-vulnerability areas are mainly distributed in the lower reaches of the Minglang river,Ganzhezhai-Hehua-Bapai and other areas with more water resources.The depth of the groundwater level in this area is small,and the water-bearing medium is uneven.The recharge of groundwater can be directly infiltrated through pipes and trough fissures.The adsorption effect of volcanic ash and alluvial mud filled or accumulated in the air-bearing zone is approximate to be missing.Because industrial enterprises are also concentrated here,the background values of heavy metals and harmful metals in groundwater are high.Aquifers are highly sensitive to pollutants or human activities,and are,therefore,highly vulnerable.The water samples with relatively poor water quality of Class Ⅲ and Class Ⅳ are mostly distributed in the areas with medium-and-high-vulnerability,indicating that the results of groundwater vulnerability assessment are basically reliable.