Coastal Ecosystem Investigations with LiDAR (Light Detection and Ranging) and Bottom Reflectance: Lake Superior Reef Threatened by Migrating Tailings

W Charles Kerfoot,Colin N Brooks,Robert Shuchman,Phu Luong,Foad Yousef,Molly Reif,Earl Hayter,Lihwa Lin,Martin M Hobmeier,Mike Sayers,Sarah A Green

doi:10.3390/rs11091076

Abstract

Where light penetration is excellent, the combination of LiDAR (Light Detection And Ranging) and passive bottom reflectance (multispectral, hyperspectral) greatly aids environmental studies. Over a century ago, two stamp mills (Mohawk and Wolverine) released 22.7 million metric tons of copper-rich tailings into Grand Traverse Bay (Lake Superior). The tailings are crushed basalt, with low albedo and spectral signatures different from natural bedrock (Jacobsville Sandstone) and bedrock-derived quartz sands. Multiple Lidar (CHARTS and CZMIL) over-flights between 2008–2016—complemented by ground-truth (Ponar sediment sampling, ROV photography) and passive bottom reflectance studies (3-band NAIP; 13-band Sentinal-2 orbital satellite; 48 and 288-band CASI)—clarified shoreline and underwater details of tailings migrations. Underwater, the tailings are moving onto Buffalo Reef, a major breeding site important for commercial and recreational lake trout and lake whitefish production (32% of the commercial catch in Keweenaw Bay, 22% in southern Lake Superior). If nothing is done, LiDAR-assisted hydrodynamic modeling predicts 60% tailings cover of Buffalo Reef within 10 years. Bottom reflectance studies confirmed stamp sand encroachment into cobble beds in shallow (0-5m) water but had difficulties in deeper waters (>8 m). Two substrate end-members (sand particles) showed extensive mixing but were handled by CASI hyperspectral imaging. Bottom reflectance studies suggested 25-35% tailings cover of Buffalo Reef, comparable to estimates from independent counts of mixed sand particles (ca. 35% cover of Buffalo Reef by >20% stamp sand mixtures).

Highlights

We explored whether 2016 Compact Airborne Spectrographic Imager (CASI) full 288-channel hyperspectral imagery could help deal with different mixtures of stamp and native sands
Reflectance imagery (3-band National Agriculture Imagery Program (NAIP), 13-band Sentinel-2, 84-band CHARTS CASI) permitted updated estimates of Buffalo Reef area covered by stamp sands, showing that cover had increased from 25–27% (2009) to around 35% (2016), that is, better than 50% towards the ERDC-EL 10-year predictions
Because migrating tailings contributed an end-member with distinctive albedo and spectral features, the combination of LiDAR and passive bottom reflectance greatly aided study of coastal shelf environments impacted by mining discharges in Lake Superior

Summary

Introduction

The second, complementary technique was bottom reflectance scanning (as MSS, multispectral scanning and full hyperspectral) This technique acquires passively reflected light in many discrete. This technique acquires passively reflected light in many discrete spectral bands throughout the ultraviolet, visible, near-infrared, mid-infrared and thermal portion of the spectrum. Our studies utilized spectral differences between stamp sands (tailings) and existing natural coastal substrates, in combination with in situ radiance and irradiance studies (Satlantic Optical Profiling Radiometer), to aid the interpretation of bottom classification procedures. In the process of bottom reflectance investigations, we encountered some problems with deep-water bottom color reflectance (i.e. difficulties during Lyzenga transformations), an aggravation facing many recent deep-water application efforts

Materials and Methods

Findings

Conclusions and Future Perspectives