Pagination and variable category

I’m trying to use links to display one of 2 categories on my page by using ?cat=15 or ?cat=4 as a filter, but the problem here is that my pagination also needs to know what category is currently active.

below is the loop I’m using:

<?php global $more;?>
<?php
// get the category ID
$cat_obj = $wp_query->get_queried_object();
$cat_ID = $cat_obj->term_id;

global $wp_query;
$args=array(
      'showposts' => 6,
      'cat' => $cat_ID,

);
$query = new WP_Query( $args );
$total_pages = $query->max_num_pages;
if ($total_pages > 1){
echo '<div id="pagination">';
  $current_page = max(1, get_query_var('paged'));
  echo paginate_links(array(
      'base' => get_pagenum_link(1) . '%_%',
      'format' => '?paged=%#%',
      'current' => $current_page,
      'total' => $total_pages,
    ));
}
echo '</div>'
?>
<?php query_posts ($query_string . ''); ?>
	<?php if ( have_posts() ) : ?>
    <?php while ( have_posts() ) : the_post(); ?>
    <ul id="cms-column" class="item">
      <li id="post-<?php the_ID(); ?>" <?php post_class(); ?>>
        <div class="post-header">
           <h2><?php the_title(); ?></h2>
        </div><!--end post header-->
        <div class="entry clear">
            <?php if ( function_exists( 'add_theme_support' ) ) the_post_thumbnail('bigger_image'); ?>
            <div class="date"><?php the_time( 'j M y' ); ?></div>
			<?php $more = 0;the_content(__('Read More &darr;', 'TRUE')); ?>
            <?php edit_post_link(); ?>
            <?php wp_link_pages(); ?>
        </div><!--end entry-->
        <div class="post-footer">
        </div><!--end post footer-->
      	</li><!--end post-->
      </ul>
    <?php endwhile; /* rewind or continue if all posts have been fetched */ ?>
    <div id="clear">
    </div>
    <?php else : ?>
    <?php endif; ?>